I agree with everything most have posted here, some exceptions based on my personal expereince..
Prior to flashing Android on my Epic ( which is a very simple process, does NOT require root)
I Was not having any GPS issues, after flashing Froyo following al the resources and reading on active posts in a few Forums, I have a GPS fix within 5-10 meters. sometimes better.
My wifi works better, i can save apps to SD card,
really digging Froyo release of Touchwiz, it is more functional and I almost prefer it to the Launcher Pro.
The Epic is Fast, snappy and works like It should. I get a full day use on a single battery charge, sometimes i have to recharge in the late afternoon if I have been streaming a lot of media, ect. I do not use my phone for the web unless I do not have access to a PC.
The camera & Screen are far better than than EVO.
I use a invisishield on the screen and it responds fine.
i really have no complaints since Flashing froyo, prior to 2.2 the phone still performed as it should. I can't see myself needing to upgrade to a different phone anytime soon.
regardless of the whole "samsung not updating 2.1" the files are availabel and can be manually installed so this isn't an issue for me & so long as I have access to those files I will always opt to Flash locally rather than do OTA updates.
The Epic truly Rocks.