I went from an iPhone4, to the Vivid, and i'm not sorry at all. In my opinion, it's superior to the iPhone in every way.
I'll cosign on this, I was a heavy Apple user and came from the iPhone 4 to the Vivid and haven't regretted it at all, that was about 2ish months ago. I always tried different phones and would last less than a couple weeks before wanting to go back to the iPhone. Last year I even tried the Samsung Infuse 4G, which was my first Android phone and it only lasted a month before I returned it. I was really leery about trying Android again, but I knew HTC made quality phones (in my opinion, better quality than more Android phones out there). Decided to give it a try, got a white one since I had a white iPhone 4, and I honestly love it.
Reasons being: I can do everything I did on my iPhone with this Vivid - FaceBook, Twitter, Instagram, MLB.tv, Netflix, DropBox, SportsCenter, Amazon, YouTube, Pandora, etc .. these are the apps I use the most. Sure, Apple has a larger app selection, but what good is a large app selection when 90% of the apps they offer I'll never download because they're useless to me. I can do everything from the iPhone on my Vivid, the only difference is it looks a lot better on the Vivid's 4.6 inch widescreen versus the iPhone's 3.5 inch retina display. Watching a Red Sox game on MLB.tv looks great, a lot better than it did on my iPhone. On top of that, the added Beats Audio enhancement (though subtle) in my opinion makes the music (through headphones) sound so much better than it did through my iPhone.
What it does that the iPhone couldn't: One word - Widgets. Enough said. There are other things, like better battery life, expandable memory, interchangeable batteries, faster processors, smoother interface and better web browsing - but meh, who's counting?
The Vivid may not be for everyone, but it sure was for me. It turned this Apple Fanboy into an Android fan. After the Infuse I didn't think I'd use Android again, but I'm glad I gave it another try. All of us will have biased opinions, but only you'll know if it is worth the switch for you, and you'll only know if you try it out. Trust me, it's worth the risk.