Count me as another recent convert. I am still in my first 30 days, so i am not totally sold. I would say I love 80% of the S2, but the 20% is just KILLING ME. Wonder if other people have these problems, whether you are a convert or not....
1. Why didn't they put the SEND button ABOVE the attachment button in the compose text screen? This is a big deal when I type in landscape mode as I have to scroll to get to the SEND button. Surely the SEND button is used more than the attachment button. That is a terrible interface design. I can live with it, but it is basic.
As for the messaging layout, you should go get something like Handcent or GoSMSPro from the market and use that instead of the native messaging app. I use GoSMSPro and love it for texting. It puts the send button right next to the text youre typing.
2. When in compose email screen and typing in a recipient, the pop up box listing suggestions doesn't fully display. It stops short of the keyboard, which I understand, but it also doesn't full show the first suggestion. Just the top half of the letters which isn't helpful. Again, terrible user interface design.
Not sure, I dont use my phone for email much (i basically use it for receiving but not sending), but again, there are plenty of 3rd party mail apps, Im sure theres one that you will feel more comfy with if you look around.
3. OS LAG big time when dealing with contacts and for the first time today calendar appointment. Took me over 10 minutes due to OS lag to create the simplest of appointments. No invitees, nothing special. The lag seems to be most prominent when I am dealing with contacts and trying to select or look-up someone. I don't think 772 contacts is that unsual, so I wonder whey this is a big problem for Android.
I havent experienced any OS lag whatsoever, Ive had my phone about 3 weeks, so Im not sure what to tell ya there. I have about 225 contacts, and havent used the calendar app for appointments.
4. No RANDOMizer in the stock music player. Easy fix by installing WinAmp, but that is a very lame oversight by Android.
You already found the shuffle 
5. In compose email screen, Attach, Send and Insert are at the bottom of the screen. When I am forwarding or replying to an email, this is a huge issue because I have to scroll to the bottom of the message before I can hit send. No fun with business emails that are long-threaded.
See above - try some 3rd party mail apps
I miss terribly the notifications customization and flexibility with the BB, but I will learn to deal with it. Bedside mode, where ONLY a phone call made any noise, was invaluable for a guy on-call every night. I can get past this, but I do miss it. This is the only comparison to BB I will allow myself as I don't want to be "that" guy. I have made the switch, but would like to get this lag issue fixed more than anything else.