If you're stuck on Sprint, and you need a physical keyboard, the Epic is the way to go. It is
extremely responsive.
Having said that, my wife and I each have one, and we'll be returning them and cancelling our service, next week.
For my wife, the battery life is the deal-breaker: The battery lasted about 9 hours for her on a day when she used the phone for 20 minutes to look something up online, and placed one two-minute phone call. She doesn't want a phone that she has to plug in during the day. (Not an issue for me.)
The screen is poor.
Go to Ars Technica for the detailed explanation. The point is that the screen is lousy for text; your eyes will continually try and focus on the fuzzy letters, leading to muscle fatigue. When I'm looking at the screen of my phone, I'd guess about 80% of the time I'm reading text (email, browser, ebooks), so I want a display optimized for text.
The only thing that the AMOLED screen has going for it are the deep blacks. I was excited about this because I thought it meant that I could use my phone as a bedside clock (LCD's produce so much light that they disturb my sleep).
But Samsung killed this dream. (Not sure why this link is broken; the URL seems correct in the editor, but the forum software seems to weirdly prepend some garbage.)
On Sprint, with the Epics, we're paying around $50 more per month, compared to what we paid on T-Mobile. Technically, we do have about three times as many voice minutes, but we never used more than 300 in a month, anyway. And, of course, we're also paying the mandatory $20/mo for access to 4G. My wife has never enabled it. I've never had need for it, and as quickly as it drains the (already weak) battery, I've always left it off.
The front-facing camera is neat. As a geek, I'll be sad to give it up. But it's really just a gimmick, right now. IMHO, it'll be at least a year before video calls are common. And, as quickly as phones are advancing, I imagine that a front-facing camera won't even be noteworthy compared to the other features our phones will sport in a year.
The issues with the GPS and the "battery full" notification, while seemingly minor, undermine my confidence in Samsung. The great thing about owning an Android device is that it just keeps getting better: Google keeps releasing updates, and new cool Apps. In contrast, Samsung's GPS "fix" provides a bogus precision, and it shoots itself in the foot with its battery full notification. (Such a simple thing, and yet it makes moot the only advantage that their screen has!)
The fact that Samsung prioritized the media hub update over the 2.2 update similarly undermines my confidence in it. It's bad enough that it shipped the Epic with 2.1; putting out its doomed media hub first shows that it's at least out of touch with my interests. (And I suspect that the vast majority of Epic users would benefit more from 2.2 than from the media hub.)
I'm probably going to wait for the G2, although there's a small chance I'll consider WP7.