I upgraded from a 5800 to the X6 (no, I didn't pay for the upgrade, I won it on a contest) and I'm torn on my feelings about it.
On the plus side now the phone won't give you alerts trough the speakers when you're using headphones. Nothing was more bugging than using fring or another IM client while listening to music on the bus and having it "pop" (while also playing your music) trough your speakers and bothering everyone else with your music and sounds.
The phone is way faster than the 5800 and feels very solid on the hand, with bigger buttons and icons since there's no more stylus. Included headphones are very nice too, finally on par with most of SE Walkman phones and so far battery and signal are excellent.
On the minus side, the keyboard looks clunky now that buttons are bigger and the text screen will show only two very short lines of text. No matter what you do, there's no way to make that text smaller. As for the speakers, they are a tad worse than the 5800 (but better than the N97) and the headphones included don't have a 3.5 connector on the remote like the ones included with the 5800, so if you want nice headphones it's either bluetooth, loosing the remote or buying the older remote in ebay or similar.
The screen is a whole other issue. The response is great, it's crisp, well calibrated (not like the yellow inked used in the 5530), but it's too sensitive. Sometimes you're typing and the screen will grab your original imput plus another one or two keys or even in your pocket your skin will be able to press some buttons. And speaking of buttons and hardware, the battery cover is a piece of crap made from a long and weak plastic that makes the whole phone feel cheap. Metal or a stronger plastic would've been nice.
Should you get it? Maybe at $300 or so, unless you already have a S60v5 phone, in which case I wouldn't spend more than $50-100 to make the upgrade, the new features are not worth it.