O.k. after officially trying all 4 of these phones this is my personal break-down ( almost nervous break-down)
All of them had their good,bad, and ugly points.
As of right now I am having the most fun with the SE p910!!
Unfortunately, this was the hardest phone to cable sync to the PC. ( Found that Norton's virus protection was the problem)
As a phone, the p910 is the easiest to dial out on-the-fly because of the normal size keys on the flip-pad. It has the loudest and most crisp ringtones, and the earpiece was the best quality.
The speaker phone on the Treo 650 was the best and typing e-mail also was preferred on the Treo 650.
The sx66 was the best 'PDA' with wi-fi and fantastic screen but was also the geekiest looking phone to hold against your face.
The P910 plays every type of sound file right out of the box and the MP3's sound pretty darn good even coming from the built in speaker phone.
The Pda function on the P910 is a bit slower than the rest but gets the job done.
I like the fact that the flip-down screen on the p910 activates the speaker-phone while your on the call. Just flip it down and 'boom' speakerphone activated.
The letter keys don't light-up for typing on the p910 which stinks but typing isn't that bad on it. It's in-between the 650 and sx66 (not as good on the 650 but better then the sx66)
Contrary to popular belief, you can do a lot of one-handed operation with the p910 using the jog dial. But...mostly when the flip pad is closed. When you flip open it becomes more PDA and you have to use the stylus about 70% of the time in this mode.
The p910 feels better against my face when talking than the other two and doesn't have a chunky antenna like the 650. Reception on the p910 has been top notch all the way with Cingular. I have had 5 solid bars for 4 days without it going down at all, where-as the other two fluctuated a lot between 2-5 bars.
The Bluetooth works best on this phone as well.
So, if you need 'phone-first' device the p910 is the way to go (no question!!) If you need 'PDA-first' go with the sx-66 (especially with the wi-fi...works great!!! Although the range is somewhat suspect)
If you need a warm mix between the two...then the Treo 650 is for you.
If the P910 had wi-fi capabilities and a lighted keyboard (stupid...because the number pad lights up but not the flip side) and a better OS...this phone would out shine them all by a large margain.