One of the reasons I bought my HTC HD2 was to try out operating systems besides the stock Windows Mobile, including Android and Windows Phone 7. I've been using various Android builds for several months now, and must say that the overall experience has been positive.
The HD2 is able to boot Android using 3 methods – via microSD card from within Windows Mobile, microSD card via a special bootloader, and via the phone's internal NAND memory. There are countless Android builds available, ranging from ultra-speedy minimalistic builds to full-featured builds based on superphones like the HTC Desire HD.
I chose to boot Android from within Windows Mobile. This has several advantages, primarily the fact that the underlying Windows Mobile firmware is untouched and no flashing of anything is required. Android is booted via a program called HaRET (HAndheld Reverse Engineering Tool) which unloads Windows and starts the Linux kernel. My two main builds of choice were Darkstone's SuperRAM Froyo (for speed) and RAFDROID HD 4.2 (for functionality).
SuperRAM Froyo booted up in about 10 seconds (yes, very fast) but was lacking in many features, like HTC Sense. RAFDROID took several reboots over several days to bring it to reasonable levels of performance (it seemed to be loading things in the background constantly), but at the moment it is about as fast as a fully-laden Android build can be.
Android is undoubtedly slower than Windows Mobile on the HD2, but remember that the phone is quite overspecced for Windows Mobile, but about par for a fully-loaded modern Android device. Despite the many lags, I am somewhat satisfied performance-wise. (Some would say it's a miracle that Android even runs on the device in the first place!)
Battery life was initially abysmal – around 6 hours, possibly 8 with the screen off. I downloaded CurrentWidget and noticed that the phone was drawing about 100 mA in standby, and that "Android OS" was responsible for most of my battery drain.
However, I discovered a simple workaround: start Android before Windows Mobile has fully loaded, and make sure the keypad lights stay on until the Android boot logo appears. Now I can make it through a full day at work (9 hours) with about 15-20% drain. If any of you HD2 Android users are having major battery life issues, this just might be your solution.
I have developed an Android launcher application for Windows Mobile that scans the contents of the storage card and displays buttons to properly launch each system on it. Additionally, there is an option to automatically launch the specified build after a delay when Windows boots up. I intend to release this to the public if there is demand for it, so if you want it, just send me an email and I'll send you the files.
All in all, I am very satisfied with my move to Android. Although I enjoyed Windows Mobile (unlike most people), there were few new apps available, and the prospect of developing for a much more modern platform, coupled with the availability of apps such as Rockbox, I knew it was time to move on. This change is almost equivalent to getting a new phone, which is yet another thing I love about the HD2.