I've had the X10 Mini for a few months now and have come up against next to no problems! I like the size of it, as I can fit it anywhere without it being too bulky, and I can use it with one hand as it is not too heavy or wide. The only problem sizewise is that when I first got it I found it difficult using the touch screen when texting as the individual letters were small to select. After a week or so I got better at it and you just have to get really accurate! The main reason I was initially put off the X10 mini was because it was touch screen. I have had 3 previous touch screen phones, all of which were Sony Ericsson, all of which the touch screen stopped working early on. These put me off Sony Ericssons and touch screens until my sister bought an iPod Touch. I decided to trust the touch screen again after using this and splashed out on this phone, whose touch screen has NOT dissappointed me.
The camera is excellent. It would benefit from more options other than flash, auto, macro, twilight and sport, as on my previous Sony Ericssons I enjoyed the features of timer, portrait, and zoom. You can only zoom after taking the photo on the X10 Mini. It is also a shame that although you can upload photos straight to facebook, you can't instantly upload videos, which I would have found useful.
There are lots of games in the Android Market to keep you busy, although slightly less and not as good as the Apple Store. I think this needs a lot of work, although some games may be unsuitable for the X10 Mini due to its size. At first I discovered that my X10 Mini kept crashing and ran slow, but this was solved after reading that I needed to download the Task Manager from the Android Market, as this would allow me to properly shut down applications, rather than hiding them. Another thing to note is that to send media by Bluetooth, again you need to download an app for it.
One of the best features of this phone is the screenlock. Instead of creating a PIN to unlock the phone, you create a patten with your your finger to join dots in sequence. I think this is far more creative and effective than using numbers.
I also like the fact that you can change the back cover, as this makes it more individual and is something I missed on my other phones.
Overall, I really like this phone, and am glad I purchased it. The main problems I encountered were solved using the Android Market, and so ultimately I have no problems.