Comparing to HTC Hero's side by side, one with the 2GB SD card that comes with the device and the other with the previously talked about 8GB Class 6 SDHC card, there is definitely a very noticeably difference.
There are still areas that will gutter slightly, and I would still recommend using apps like TasKiller to keep the quantity of memory resident apps down, but when it comes to loading up or searching through contacts, albums, music, emails, text messages etc, there is a considerable amount of difference in responsiveness.
If you have the Hero, I highly recommend getting an updated card. However, I do also hope that HTC will release updates to HTC sense to streamline some areas that obviously have room to improve.
As for any other problems. I've spotted one or two very minor bugs that most users probably won't even pick up on, but the only big issue I've had is the IMAP support with the HTC Mail client. For the most part it is fine and works well, but I just happen to use hMailServer on my servers and just happens that HTC Mail client and hMailServer aren't playing ball. Luckily hMailServer has a very good lead developer (Martin) that seems keen to try and get the problem resolved.. Would be important I guess to get that done considering HTC is planning to replicate the whole HTC Sense software across their entire Windows and Android phone portfolio!! *fingers crossed* that will be sorted out!
What do I think of the device? Amazing.. I've used everything from Nokias, iPhones, Blackberrys and various other HTC windows devices, and this one really just feels right. There may be tweaking to do, but its just way ahead on the usability of any other devices in its market.
Anything I don't like? Probably only one thing if I was pushed for an answer. I wish that more of the apps developed for Android would close properly and take themselves out of the process and memory space when they simply have no excuse of being there. Android has the ability for more than one app to be running at a time, but I don't like developers getting lazy or exploiting the fact. Thankfully there are apps to help, but just means something else to do just because these people couldn't be bothered. The only other thing is the lack of BlueTooth stack support. Being able to push files and contact information would have been very useful. Lets hope for something in the Android "Donut" update.