Music equalizer is just a selection of 10 pre-set types; jazz, folk, flat, normal and etc. USB and WiFi tethering work well. USB tethering can make use of local WiFi or 3G. Had Bluetooth tethering via PdaNet before the upgrade but not really required now. Downloaded Adobe Flash 10.3 and used it with Opera Mobile. Seems to work OK. Opera Mobile has options to manage how Adobe Flash is used.
The only thing I've found is that I can't configure Vodafone email clients any more. Other sites work well with the Android V2.3 mail client so who knows what the issue is with Vodafone.
USB charging is really slow. hours of charging and you've only charged the battery by 20-30%.
Haven't had any problems with any real lag in performance. I moved a lot of my apps onto the SD card and this may account for some lag as things are cached.
It has been mooted that a 32GB SD card can be used with the XPERIA X10 however there has also been reports that some cards/phones don't support 32GB. Be careful. 16GB is supported.
My applications which gave me access to my NAS and printer all seem to work.
So all up, I feel like I have a 'real' mini computer in my hand. And given I paid todays market price for a new XPERIA X10i a few weeks ago I feel I'm not to bad off.
If you paid full New Zealand price for your XPERIA X10 when new, you may now feel you've got some return for your outlay or at least get something for it on trademe.
I ended up flashing mine to a custom rom last week as the update for my version wasn't out. Works well, simple painless experience. Phone seems to be a bit snappier, and it's nice to have things like flash working.
Kinda wish I hadn't installed a custom rom now though, I seem to now spend half the day looking through roms and themes and trying different stuff out :( haha