I made the switch as well. It has been a hard decision but the phone is starting to grow on me. I became very well versed with the HD2 and it's nuances. I started with Cookies Home Tab mods on Winmo 6.5 to learning how to mount and unmount APKs and scripting with 2.3.7. The HD2 was my journey into Android modding. Funny enough, My first smart phone was a G1 but at that time I wasn't into and didn't know what I could do with android. Coming from Symbian, I was like, hey, it just works.
As time passed though, I was promoted to be in the field as an I.T. consultant. The HD2 became more of a functional necessity rather than just a cool phone I could mess with from time to time. With that said, things just had to work... and at times, had to work fast and also multi-task better. The HD2 started to feel its age. I needed a phone that was an all out improvement over the HD2 so I wouldn't look back.
Unlocked boot-loader, larger screen with higher resolution and brightness, faster processor, faster data speeds (and even better signal attenuation), lack of hard buttons (wear and tear), lighter phone (higher chance of surviving a drop) and just all around newer technology. And the best thing for me was MORE AVAILABLE RAM
I was happy I made the choice. With a good ROM with the sgs2, it's simply graceful. With the increased RAM and performance, I can do everything a lot easier. I still have my HD2 in pretty decent condition running ICS but it's just for kicks. The Tmous version of the Galaxy S2 is very young so with that you won't get nearly as much legacy knowledge or development as you do from the HD2 subforums. I'd say give it time but with the available ROMS, they have this phone very solid.
It's an all around better phone in every aspect. Even the audio is better and louder. The ONLY thing I miss is a notification LED but I'm learning to live without. HD2 owners should already know the LED notification issues we've had with previous ROMS as it is.
A few things you'd have to get use to in the change. The phone (without a cover) will fit anything that you used to hold your HD2 in granted the HD2 had a cover. BUT if you decide to get a back over for the phone, you'll need something else to hold it in because it's larger as it wider and longer. The audio plug port is now on top instead of the bottom so if you had a car setup, you might have to move things around a little. The charger is a micro-usb port and it's almost in the same location as the HD2. The power button is on the top right side of the phone so that will take a little getting used to. It does have a 2MP front facing camera so that's another bonus. Without a cover the phone is still easy to hold and doesn't feel like it might slide out of your hand. It's quite light for it's size and thin. The buttons on the phone are capacitive with the exception of the volume rocker (on the upper left of the phone) and the power button. The speaker is on the bottom left of the rear of the phone but it's quite loud. 1 LED flash as opposed to 2 with the HD2 but still seems to put out the same amount of lumens. The camera is more recessed than the HD2 and is flush with the phone body so no more scratched camera frame. The SD card IS hot swappable so no need to remove the battery. The plastic back cover is very thin so take care in taking it off and on (no brushed aluminum like the HD2). It's a lot but that's pretty much all I can think of for now. Hope this helps.