I've had an HTC HD7 for about 9 months and I've been running the Mango update for about three of them.
App support continues to grow because MS has a really good relationship with developers. They actually teamed up with the guys that first jailbroke a WP7 device to offer the service to others:
http://labs.chevronwp7.com/That said, to be perfectly honest, since I put this new update on my phone I haven't really had a need for many apps. I think that's pretty telling considering I'm always attached to my phone and I use it for everything I do, including work. The only third party apps (aside from games) I have now are: Kindle, FourSquare, NFL Scores and a finance tracking app. Everything else that I need from my phone is pretty much taken care of. I used to have a Twitter application called Rowi, but now it's built into the OS, so...
I especially love the upgraded Zune Pass integration, and it's unfortunate that Engadget didn't really make mention of it in the review. Right now I can snap a picture of an album cover and download it, in its entirety, directly to my phone without any kind of hassle. That's awesome.
Edit -
Web Marketplace is up now, so you can check for any apps you'd want