I bought a 64GB iPad thinking that I would need it. Much to my surprise, I only use 6.5 GB. I don't have any music on it because I'd prefer to use my Sansa Clip or DroidX for stored music, but the bluetooth radio is really solid. The range is great and it's very clear so I usually use a
BT headset instead of plugging in headphones. I can have the iPad in the living room, walk 2 rooms over and still be connected. I also don't keep a bunch of pictures on it.
What I didn't realise is how much I really enjoy using the cloud. I have Carbonite backing up my entire desktop and I also use Dropbox. Whenever I need a file of any sort, I just go into the Cloud and download it onto my iPad. I like that my iPad isn't all junked up w/ files and am quite surprised that it isn't.
When I bought it, I thought 64GB would be too small, but I'm pleasantly surprised that it has been more than enough. I didn't even realise how much I relied on the cloud, but am totally loving it!
I agree w/ the others to get the biggest one that you can afford and then see how you use it. If I had bought the 16GB I might feel like I'm restricting myself, but now I realise it's just how I naturally use my iPad.