I started out without any problems and then after downloading way too many applications, I started getting an error message - something to the affect of "Your application area is getting full. Please delete some applications or move them to the card." The first thing I did was check the phone and card space and found that there was 6gb of space left on the phone and a couple of 100mb of space on the card (books and music mostly).
At this point I'm not sure what is going on, so I called Verizon Wireless support. They eventually told me I would have to call HTC support. When I spoke to HTC, they told me something I should have figured out already. Of the 8gb internal memory less than 2gb were available for use by the user. The rest was reserved for use by the OS and the built-in apps (at the time I was using a 3rd party mail application that was restricted to the user area, by switching to the built-in mail app I saved a significant amount of space and was able to use some reserved space).
I felt used to say the least. I really only had 4gb, at best, of usable space. At the same time I bought my phone they had been offering the Droid with a 16gb card (16+2 vs 8+2 was okay, but when you factored in the reserved space it was a different story). I decided I not only wanted to complain, but wanted to go back to the store and voice my dissatisfaction in the hope that they would comp me more memory. I did exactly that and, after some back and forth in which the Verizon rep kept saying it was HTC's fault not theirs, the manager agreed to give me an 8gb card.
I'm much happier now. I believe they should have figured this out from the beginning, but that what I get for getting the phone sooner rather than later.