Here's my experience. Please note, this is my first Android phone...I moved from many years on the Palm platforms. YMMV, of course, and these are just MY opinions, based on MY wants/needs. I am NOT saying one is better than the other, just that one is better for ME than the others.
I looked at all three myself a week and a half ago. Actually, I just looked at the Epic and the EVO....after talking to the salesman, he recommended I take a look at the Optimus. I had two complaints with the Epic and the EVO...first, the 4g monthly fee I would be forced to pay, when I don't live near a 4g area, and I have yet to see a 4g rollout plan for Orange County, CA. Secondly, the phones are just a little too big for my tastes...but that is just me. Plus, most of the features that would drive a user to those phones, really didn't interest me a whole lot.
After playing with all three phones, I actually went with the LG. First, the LG, to me, is extremely responsive..I don't notice any lag moving from screen to screen, launching apps, etc. Everything works wonderfully for me. The EVO is a BEAUTIFUL phone...and if I wanted to watch movies on my phone, I would have gotten it. But I have a nice iPod Touch I use for movies. I don't intend to watch TV on my phone, so the screen didn't matter a lot to me. For what I wanted the phone to do, the capabilities of the LG for just fine.
Second, from the moment I held it in my hand, it just plain felt comfortable. The EVO was a hair too big for my taste, and the Epic a little too "bulky" feeling. But the Optimus fits my hand, and up to my ear, the best. I usually use a bluetooth, but for those times when I don't, the phone works great.
Now, the only downside I have noticed. The soft keyboard, even when in landscape mode, is smallish (I'm 6'3", and have bigger hands), and it makes me wonder if the added size of the EVO or the slide out keyboard of the Epic would have made it easier to type. I do a lot of texting, so it's kind of a big deal to me. Having said that, I am getting more used to the keyboard, and as I'm getting used to it, I'm gaining speed plus accuracy. But it's been a bit of a curve for me, though I am noticeably better after just 2 weeks.
Overall, I love the LG Optimus S...I'm still within the window to take it back and exchange it, but honestly, other than the soft keyboard, which I'm getting used to, I really don't see why I would want to change. PLUS...the price was a HUGE difference...the Epic and EVO are hundreds, while it cost me about $70 to walk out the store with a phone that fit ME perfectly, and does everything I need it to do.
I would suggest going to a Sprint store, and doing what I did...play with all three side by side, to see which fits he best. Like I said, the EVO is a BEAUTIFUL phone, as is the Epic...but from a price/performance perspective, I think the LG is a great little phone.
Hope that helps.