iPhone convert here. Been through the iPhone 4, 4s, 5, 5s. Love this phone. A few thoughts from me below:
The good:
Screen size, screen quality (yes, to me it's noticeably sharper), battery life is better, the ability to use widgets to see information at a glance, you can customize everything. The hardware build quality is on par if not better than the iPhone. Having NFC and Google Wallet, to me, is a nice perk over the iPhone as well. The pull down notification screen is much better than iOS in my opinion. I like the LED notification light. Sort of reminds me of my good old BlackBerry days, and it's nice to be able to glance at the phone to see if you have a notification waiting.
The negligible:
Apps don't always feel as polished as iOS. Some feel like an afterthought. Due to the different Android phone screen resolutions, since you're coming from an ecosystem with a defined screen resolution, you'll notice some oddities from time to time.
The stock mail app sucks compared to iOS. It doesn't auto-resize e-mails for whatever reason. I use Google Apps for business so I use the Gmail app for everything, so for me that's a non issue, and it will be a non issue for most. I'm OCD though.
There are no lock screen notifications. The app "NiLS" with the floating widget, a purchased theme, and some customization fixed this for me, and now it's like what I had on my iPhone.
OK, some will think I'm reaching a bit here, but I like to have my Visual Voicemail in the phone dialer - like in iOS. I'm on Verizon, and they wanted me to pay $3 for a service that's free when you use an iPhone. I said nope, and set up Google Voice, which is working nicely. You can do a conditional forward to your Google Voice number and it works well (download the app).
The phone is substantially heftier and larger than the iPhone (duh). Try it at the store. I see a lot of butthurt Apple Fanboys who are irritated that Apple may be increasing the screen size of the 6, but since you're considering switching, you probably aren't one of those. My Apple Fanboy hangs low. I still left.
The carrier bloatware is very irritating, but can be disabled. To me it's still a negative but nothing to keep me from loving the phone.
The bad:
After using it more, I've decided the camera sucks. HTC could have done much better and it's dissapointing they didn't. You'll see people arguing both ways, but if you're coming from a 5 or 5S you'll miss the ability to point and shoot and not have to worry about it.
Another OCD thing from me. I hate the USB charging port. I liked lightning cable that was on the iPhone; you didn't have to worry about which side was up. Really nice if I've had too much vodka and go to plug in my phone, haha. Again, to 99.99% of the rest of the population, they simply won't care.
Otherwise - it's a phone. It does the same stuff, in a slightly different way. I've tried switching before, and couldn't pull it off. Android is much more polished than in the past. I think the phone is great, and do not plan to return unless the iPhone 6 is freaking spectacular.