I have an i5700 and it's not bad at all. It's no laggier or slower than any of the current T-mobile offered range [G1/G2] and it has a capacitive screen, which is nice.
It also has the full range of accessories you'd expect - accelerometers, AGPS, magnetic compass, Bluetooth [with the usual android bluetooth restrictions - google 'em], and it's the first Android phone to support XVid/DIvX out the box - up to 720x480 IIRC, so less tedious re-encoding to do. Although Handbrake used to convert stuff to 400x2xx works nicely if you don't mind ripping stuff.
I did have an S60 3rd edition 6650 fold, an the i5700 makes it look positively archaic - really, really crap - in comparison.
Also, £20/month-18moths for 300/unlimited/unlimited calls/texts/web [fair usage applies, as ever - 1gb, but wifi gets around this] is not a bad deal for such a good phone.
The only problem I have with it is that I wanted to flash it from Android 1.5 to Android 2.1 [Samsung 'accidentally' released a Beta ROM - see
here and the rest of that forum for details] but it only works reliably on Win32 platforms as it requires drivers to pick up the hardware in firmware dump mode - I have Win7 64bit and it just doesn't want to install the drivers for it at all. I'm just going to wait for Samsung to release an official patch, or for T-Mobile to put an over-the-air upgrade on it - rumours abound that it's only a couple of months away at most.
I've never used an X6, but if it's based on Symbian, I can't see it being much better than the N95/97 et al which I was never particularly impressed with.
if you can hold your horses, the HTC Bravo [AKA Nexus One spec phone without the Google branding] will be out on T-Mob in a couple of months - it might be worth waiting for that.
If you are just bored with your Nokia though, and want to try something different, an i5700
can be picked up online for about £200, which I reckon is not bad at all for what it does.
It might be worth noting that you can't do multitouch on teh i5700 - allegedly the touchscreen controller needs flashing to enable that, but I haven't heard of anyone doing it successfuly - pretty sure th G1/G2 support it with the right minor tweeks however. If that sort of thing is important to you....
Also, if you have a Google mail account, you can import/sync your contacts from that, which wors really nicely once you get it set up - export your existing contacts as a CSV, upload to Google, put them in My Contacts and do it. If not, a company called
Sprite do a reasonable contacts transfer app that works [with a bit of fiddling] for that too.
Overall I'm really chuffed with the i5700 - it's not as ultra-doublebuffered smooth as an iPhone, but then you can have third party apps running in the background on Android, something an iPhone won't do, so you expect a bit of slowdown when its working, but nothing major.
I'd strongly suggest going to a large T-mob store and asking to have a look at one.
It's a perfectly usable, highly functional phone - last night, i SSH'd into my Linux box and downloaded some iPlayer content with it. From my bed. I then move it to a network share, copied it to th SD card, and watched it. I could have VNCd to the linux box from the phone if I wanted to...very versatile wee thing.