I picked up my Xperia X10 a couple of weeks ago for a penny. At that price, what was there to lose for a secondary phone to play with Android on? (This is my 2nd Android phone, my first being an HTC Aria that I wound up giving to my daughter-in-law).
Out of the box, the device was hardly awe-inspiring except for the screen & its size. Running 1.6 with Timescape and Mediascape was painfully slow and the underlying OS rather creaky. However, the big joy of Android is customisation, and the 1st thing I did was to debrand it to free it from the shackles of AT&T and then I flashed Android 2.1 directly from Sony-Ericsson. The 2.1 update was MUCH better, but honestly, Timescape and Mediascape still dragged a bit (although not nearly as bad as under 1.6). Also, multi-touch was enabled with the 2.1 update and the device really was is a quite a bit more usable overall. Of course, AT&T has already discontinued the device (after less than 8 months!) and they never even got the Éclair update out, so anyone still branded to AT&T can forget about the Gingerbread update from them.
Honestly, I could live with the stock 2.1 experience, especially if one doesn't bother activating Timescape which seems to be the biggest drain on the device's responsiveness (Use Root Explorer and get rid of it and the other SE stuff you don't want). But where's the fun in living with a stock ROM?
I'm presently running a CyanogenMod ROM based on Froyo with SPB's Shell 3D and loving it. Performance is great and I'd put it in the same class as some of the better devices currently out there. Customisability has made all the difference in transforming this device into a first rate one.
Now that the bootloader has been cracked, I'm keeping a close eye on what the devs will be cooking up next, but I have to believe that it'll only get better from here. Since this is a secondary phone for me (my iPhone 4 is my primary), I can play to my heart's content.
Since I'm debranded, I will be able to get the Gingerbread update when it comes out early this summer and naturally I'll snag it and play with it. But I'll still keep an eye out and see what the devs at XDA-Developers are cooking as I'm sure they'll do some great things with the Gingerbread ROM when it comes out.
I know there's a lot of hate going on for this device, but honestly, it's Android, so the hate is unnecessary. Root the puppy and make it what YOU want it to be. This is truly the one aspect of Android that I absolutely love. You're not stuck with what you're delivered and, really, rooting and loading custom ROMs is pretty drop-dead simple with Android (I did the same thing with the Aria for the brief time I owned it). So yeah, I'm sticking with this device and will continue to mod and play with it to my heart's content!