I had an HD7. It just broke last week. (not the phone's fault, FYI) I decided that instead of replacing it, I'd get the Nokia Lumia. So far I am in love with this phone! It makes my HD7 look like an asshole. Side by side, the screen of the Lumia instantly distances itself from its cousin with a beautifully color saturated and drastically brighter display that even Helen Keller would notice right away! Lol
Also, the difference between 3G and 4G LTE is worlds apart. The rest of the WP functionality is there, the way MS wants it. Facebook, YouTube, Zune, Xbox Live, etc. integration is all there!
Another good thing is the price point. $99 with contract is a pretty good deal for a brand new 4G phone, so it really is a good entry level phone!
I haven't had mine for very long, so I can't really tell you any downsides. If you are set on wanting a Windows Phone, the Lumia 900 is pretty much a sure thing.