I am a Treo 180g user who has been carrying a T600 from Sprint for just a few days now. Test driving it for the execs at our company and here's my take on it

Screen is REALLY bright! In fact, if I'm in a public place and it's dark I actually turn down the brightness.

Battery life is a huge improvement from the 180g (duh). With moderate phone time and heavy web/email usage, I get about 20 hours out of it. I'm sure that heavy talkers will not find that to be enough, but for me it works fine.

Browser is zippy on Sprint's network, though the small font size is sometimes really hard to read.

Five-way thumb controller negates use of the stylus almost 100% of the time.

Sound quality is clear. Even speakerphone is improved.

Downsides:
Losing grip on the thing... I haven't held a PDA-type device that felt like it belonged in my hand since my Palm Vx. This is a very expensive device to have it feel slippery in my hand (ditto every Treo on that). I've already dropped it a couple of times but it survived.

Center button on five-way controller.... sometimes I miss the button with my thumb and hit either up or down. I find I'm using my thumbnail more often.

Keyboard... Can't fault HS entirely on this. It is probably the best keyboard they could fit into such a small space. I am getting used to it, but if you have big mitts you're going to find it difficulty initially, no doubt.

Silent-mode swtich on top of unit.... This is incredibly annoying . Probably the worst designed part of the T600. I find myself fumbling with the swtich trying to slide it one way or another. You have to actually hold the unit with BOTH HANDS to get a grip on the switch. One-handed use is almost impossible. Also, do you think HS could have put an icon on the screen somewhere to let you know you were in silent mode? Would be very helpful, especially in a dark movie theater. Like my 180g, I'll have to create mnemonic devices to remember ('L'eft for 'L'oud). This seems like a 'duh' addition to the T600 that they missed.

Camera... awful but also a lot of fun. I've taken about 50-60 of the grainest, darkest, "last known hostage photograph" pictures in the world. I'm sure HS could have put something else cool into the T600 other than a camera, but I still use it once in a while.

Battery indicator... a fully charged battery (solid green LED) appears like a partially charged battery on the LCD indicator. I've never trusted the battery indicators on Treos. If I'm talking on my 180g and get warned that battery is going to die, I end the call only to find that after I hang up the battery magically increases in power. Evidently the Treo is threatening me. "If you keep talking like this I'm going to die on you!"

All in all, I like the T600 and can live with or adapt to its flaws. I am likely to with one once T-Mo supports them.