Well, after the first night I can say I have mixed feelings, but overall I am happy with the Centro. First the negatives:
1) I do miss the flip form factor of the i500 for a variety of reasons, notably the screen protection because I always keep my phone in my front left pants pocket, and also because the angle of the i500 when flipped open feels ergonomically better.
2) The earpiece volume while calling seems rather low, even when the volume adjustment is at 100%. The speakerphone seems to work well though. I just downloaded VolumeCare 6 Pro to try, $19.95 to buy, from
PalmGear.com (
VolumeCare 6 Pro - PalmGear.com: Download Palm Software for your Treo 650/600, Tungsten, LifeDrive or other Palm OS Device) to address the issue, though I'm not sure if it might be a problem just with this particular phone. If not,
Palm or Sprint ought to issue a software/firmware/hardware update because I shouldn't have to buy extra software to hear people calling me.
3) Battery life does not seem to be very long, of course I have been browsing and chatting much more than usual since the newness factor has not yet worn off
4) As mentioned in practically every review, the stylus is of pathetically poor quality. Yes, this is a low priced phone but the stylus shouldn't feel cheap to the touch or flex unnecessarily under use.
5) I haven't used the microSD slot yet, nor do I expect to be changing it very often when I do, but to access the slot one must remove the battery cover which seems tedious.
6) The camera, which has no lens cover, is placed exactly where my index finger tends to be while holding the Centro one-handed with my right hand.
7) The phone has GPS but only for e911? It seems odd that this feature would not be available for use with included programs such as Google maps. Perhaps a hack could open it up?
8 ) OS5 Hacks, or lack of hacks: I am slowly understanding the differences between OS4 and OS5 and apparently OS5 is extremely less hackable? Perhaps you more experienced users can chime in here.
Seems like a long list, but now for the positives:
1) Screen resolution is very sharp and very readable.
2) Device size is pretty good, perhaps the screen could be a little bigger without enlarging the device dimensions, but see #1.
3) The QWERTY buttons are fairly easy to use and already I can type much faster than I could ever write on the i500 with the stylus. Occasionally, I find myself trying to type in the eZText (like t9) mode that the i500 had, but with practice that should go away.
4) 5-way navigation button: Works surprisingly well in combination with the keyboard, so you can do most things one-handed without the stylus.
5) SMS: I really don't need to say anything about that other than it works, beautifully.
6) pTunes: I only transferred a couple songs from my computer to the Centro, but no transfer problems and the sound out of the built-in speaker is not fantastic but decent. I haven't tried the headphone jack yet, but who knows when I'll get around to getting a 2.5mm adapter

.
7) IM: I used it today to chat with a friend in Italy - she was surprised that I was online at 9:00 EDT since I can't use Windows IM from my work computer. This is not an essential feature but its brilliant to include it for the target market.
8 ) Sprint TV and On Demand: I have the $30 Vision PRO pack which has unlimited access to data/web/Sprint TV and unlimited text messaging - you want unlimited text messaging if you plan to use IM since each message is actually a text. These 2 features are other nice to have "diversions", I don't anticipate using them often but if you're killing time at the airport or doctor's office they could keep you entertained.
9) Google Maps: seems to work well; you can view the street maps or satellite views just like from your desktop. Also, it will display live traffic and of course give you directions, but unfortunately no live GPS.
10) $99 price, well $199 with a $100 mail-in rebate.
There, more positives than negatives!

I'm sure that as with the i500, the ingenuity of software developers and forum community can address some of the negatives. Keep in mind positive point number 10 compared to the $400 price I paid through Amazon.com AFTER a $200 discount for my first i500 in 2003.
That's all for now, so commence the replies...