I have a Pre 3 and a N9.
The N9 hardware is very nice, a beautiful phone, except the volume and the power keys, they feel cheap and wiggly. Nokia includes a rubber case, which is very nice. Compared to the Pre 3, the N9 has NFC and a FM receiver (transmitter is in the chip, but there's no antenna, so currently no way to use it). Pre 3 of course has a hardware keyboard.
The best thing about Meego Harmattan is the Swipe UX. I am a huge fan of gesture based UIs and the N9 has zero buttons on the front, not even a gesture area
If you're a heavy Facebook or Twitter user, you will like the Newsfeed screen. RSS feeds and calendar entries (with a 3rd party app) can also be incorporated into the Newsfeed.
Meego Harmattan has a system wide Accounts app and additional accounts, for example ICQ, can be added easily (there's an app in the Ovi/Nokia store or with N9QTweak), unlike webOS' Synergy which unfortunately has virtually zero plugins. Available accounts include CalDAV, Mail for Exchange, Google (including Picasa and YouTube), DropBox, Facebook, Twitter, Skype and Flickr.
Harmattan pulls contact information from these accounts (where applicable) and combines the information for your contacts (of course you can (un-)combine contacts manually too).
Unlike webOS, Harmattan does not combine messages with a contact over different IM systems into one conversation, i.e. in the Messaging app, I have seperate Skype, ICQ, GTalk sessions, all with the same contact.
We all already know that, but notifications are far superior in webOS New notifications are shown as an icon in the status bar, but you can't do anything from there. They are also shown in the Newsfeed. If you tap on one the corresponding app opens. You can't remove individual notifications (there's just a remove button for all), let alone swipe them away, which is disappointing for an OS that heavily emphasizes gestures. One of my favorite webOS patches is the 'delete email from notification' patch. You can't do that in Harmattan, so I have to swipe to the Newsfeed, tap on the notification, wait for the Mail app to open, then tap the delete button and close the Mail app (or just leave it open). Notifications are also shown on the lockscreen, from there you can swipe the notification and the app opens. Only thing that's better regarding notifications in Harmattan is that pending notifications are not 'in the way' on the bottom of the screen. On the other hand you can access the notifications from everywhere on webOS.
Besides the lockscreen, there's also a standby screen which permanently shows the clock (and additional information, with, for example, lpsmagic) and non-interactive icons for notifications. I don't know if I like it, for my taste it's too bright when it sits on my bedside table at night. The clock also moves around and I have the need to look at my N9 whenever I see something happen on the screen from the corner of my eye. Kinda disconcerting. The standby screen can be turned off completely.
As someone said before, even in the Mail app you can't swipe to delete mails. Tap and hold to open a menu. Feels inconvenient, especially compared to webOS. Tap and hold to open menu is in a lot of places.
I never liked the webOS browser. The N9 has, additionaly to the standard browser, Firefox and Opera. I like Opera, it's very fast and, unlike webOS' browser, it reformats textblocks so that you can read without scrolling, scrolling, scrolling. Flash is also available, but only for Firefox. I haven't used it very much, but it kinda works if the Flash content is not too demanding.
The app situation is comparable to webOS. Harmattan is already pronounced dead by Nokia, so it's unlikely that it will ever have a huge developer following. Anyway, there are quite a few apps in the official store and there are 3rd party app catalogs, like apps.formeego.org, n9-apps.com and my-meego.com. The community at talk.maemo.org is also very active, but the webOS community, especially homebrew, has a much better infrastructure and is organized better. Thanks to webOS Internals. Don't get me wrong, the Maemo/Meego community is great in its own right, but for example there's nothing like Preware (yet) or at least community maintained repositories (like for the N900) for homebrew. But I think it's only a matter of time until the N9 gets something like FapMan (a Maemo package manager) and proper centralized repositories for homebrew apps and patches.
Some apps worth mentioning are gNewsReader (a proper Google Reader client), QuasarMX and SigmaPlayer (great audio player apps), SportsTracker, AGTL (geocaching), CloudGPS, QmlReddit, NFCinteractor, MeeScan (QR code scanner). There are a lot more, have a look through the links above. Nokia also provides free maps and navigation. I don't use it, but I heard good things.
I see a problem that I want to call 'OS fragmentation'. Harmattan only has the N9/950 (and btw it's not even proper Meego, more like Maemo + X). There's the 'official' Meego. It is too early to say anything about where Tizen is going. There's Mer and Nemo. Symbian is still huge (I mention it because of QT apps). Fremantle is still alive, too. My point is that some apps that work on one platform may or may not or only partly work on another platform. And the UIs are not really unified. Harmattan has developer and UI guidelines and is still supported and updated by Nokia, but after that? And what when Tizen is ready, will it even support QT?
This actually is not a problem at all, just idle thoughts and something that, to be honest, bothers me a little bit. There are forks and ports and whatnot left and right, it's a little confusing, especially for devs. Anyway, the N9 is current and supported, has apps, great SDK and APIs and works fine right now.
The virtual keyboard is very good, I like the feature that you can swipe on the keyboard to change to another language, which is very useful for me, because I sometimes write in English and sometimes in German. It also has a nice click-sound and haptic feedback, both discreet enough that I enjoy it, where I was annoyed by it on other platforms. Swype is also available and works fine.
I want to add that Harmattan has not yet Skype video calls, only chat and voice calls. GTalk video calls can be enabled.
What I like about Pre 3 / webOS and what's missing in Harmattan:
Just Type, it's just so very convenient. N9 has a Search app, but it's very, very basic compared to JT.
Exhibition Mode. The N9 standby screen is poor compared to Exhibition mode.
Superior notifications. Combined conversations from different IMs. Cards metaphor works really well. Best of both worlds would be great. No gesture area, just swipe from the very bottom up to open cards view.
Of course there is no need for rooting or jailbreaking, just enable dev mode and you have a terminal (BusyBox by default) and root access. There's the AEGIS security system, which has already been 'cracked' with Inception.
There also already is NITdroid, an Android port, currently ICS 4.0.4, which is very promising. I think there will be a completely usable version released someday.
My big, big hope is that one day I can install Open webOS on my N9. I really like Harmattan, but webOS is far more mature. But Harmattan is still young, and a lot of features will still be added, either by Nokia or by the community. Hopefully more gestures, like swipe individual notifications away, or swipe to delete mails, conversations, etc.
Excuse this wall of text and have a nice day! And if you have any questions, ask away