I've had an HTC s710 (sold as a Vodafone v1415) for about a month and a half now. After seeing some of the bad ratings the phone got, I felt the need to write my views - which in short are that I think the phone is great.
The features I use the most on the phone are its calendar/diary function and Wi-Fi internet. The diary function works in a very similar way to the one on Outlook, and is easy to use. It allows the normal system of viewing by month, week or day, with the ability to set reminders and notes to each appointment. It also has the facilities for replying to meeting requests or creating them in other peoples' Outlook calendars, in the same way as the PC version can.
The phone does not support 3G, which is a bit unfortunate meaning the internet can be a bit slow - unless you're in a Wi-Fi zone. It connects without any hassle to Wi-Fi networks (b or g) including ones with password protection such as WEP or PSK. I regularly use its Wi-Fi to check emails when at home (saves me having to turn my computer on) or at friends' houses. It supports imap and pop3 email accounts directly, making it easier and quicker than having to use the internet browser to go to the web-access site.
Both email and the calendar synchronise easily with Outlook on your computer. This makes it easy to avoid having the problem of two incomplete diaries, such as by having a paper diary that you carry with you and another on the computer.
The MP3 player it comes bundled with is Windows Media player, and the amplifiers in the phone seem to work well giving good quality stero sound. The external speaker is loud without losing too much in quality.
There's one odd feature with the phone in that the way to close programs is not obvious at first. Returning to the home screen does not close them; you have to go through the Task Manager to do that. While this means you can have multiple programs running at once, it also means that you can inadvertently clog up the memory. Closing programs with the Task Manager is not difficult to do since the phone can display a list of the 10 last accessed programs on the home screen and once you get into the habit of it, the Task Manager features regularly there.
I have not had any problems with the phone crashing or temporarily freezing, though I do agree with other reviews that it can be a bit sluggish when switching from portrait to landscape display when you pull out the keyboard.
Lastly, the phone is not expensive. I was able to get the phone for free with a contract paying less than £20 a month.
In short, it's a fantastic phone. It's great for integrating your diary into your phone (so you don't have the hassle of carrying two things with you), making it easy to check emails when not at your computer, oh and making phone calls too.