Summary: I've had this phone since October 2007, and i bought it second hand. I have only had one problem so far - and that was because i had it unlocked, occasionally it rejects my sim card, but that is solved by a simple turn off and back on again.
Summary: Windows mobile is powerful and extensible, but out of the box for a phone or even PDA-based user it can be a bit difficult to use. Microsoft have got to work harder to get up with competition such as other PDA/phone built-in applications.
Pros: Keyboard, Windows/Outlook sync, Wi-Fi, Media player, voice-activated dialling, miniSD for media and files, GPRS for mobile based surfing.
Cons: Slowish CPU and/or short on internal memory. Slow O2 software.
Summary: Just replaced a treo 650 with the O2 XDA Mini S. As a Mac user, I've always leaned towards Palm-based phones as they're easy to sync with Mac OS X. However, combining this phone with 'missing sync' software (about £20) was a breeze - first time sync, no problems.
Summary: I have never gone more than 3 weeks without either the phone doing a hard reset itself and wiping all my data and software, or me having to do a hard reset because the software became corrupt and only the essential functions would work (this doesn't include the phone).
Summary: The O2 XDA Mini S is in short brilliant. Its new operating system is its core feature, which blows Palm away. The keyboard is extremely useful and makes text-messaging, MSN, email and data-entry actually viable on a handheld.
Pros: Instant, out-of-the-box functionality; Wi-Fi; intuitive, powerful operating system; wealth of features; hi-performance after O2 'software' removal
Cons: apparent slight lack of built-in memory (dependance on new SD-mini cards),
Summary: All round good phone/PDA. Was initially concerned about the O2 ROM as I had heard that there were bugs which cause the device to crash. However I think that there is a new ROM which is installed, my device has no problem. It works well, TomTom works too (with Bluetooth GPS reciever)!
Pros: Most of it
Cons: Whilst listening to MP3's the keypad and screen both can't be turned off, using more power.
Summary: In theory, this looks as though it has everything I want, but using it day-to-day has been a nightmare. It's very slow and crashes frequently, especially if you have more than one app running (and they don't close when you think they have).