I was extremely nervous to purchase a mattress online, but because of the mostly great reviews, Walmart's outstanding return policies, and the fact that NOTHING could be worse than what I was currently sleeping on, I went ahead and took the risk.
The first 12 hours or so had me reeeally worried... The mattress didn't pop right up to it's full 8 inches right away as some reviews claimed, and once it started expanding there were weird harder places where it had been folded that I worried wouldn't go away. We did not sleep on it the first night, but let it air out and expand undisturbed. I also took the cover off and washed it so that the foam could air out even better and to eliminate the odor soaked up by the cover itself (which wasn't too bad at all and was unnoticeable after a few days). It washed and dried with no problems, but was sort of awkward to put back on. No big deal. By the next morning the mattress was smooth and measured an exact 8 inches! I think some may expand quicker than others, which is probably related to the amount of time it spent squished down in the box before being unfolded.
The first night we slept on it, it was comfortable but a little firm for our personal preference. Also, because we each weight around 240 lbs., the mattress was not quite thick enough and we could sometimes feel the box-spring underneath. This only happened when we were standing on our knees on it to test out the thickness though anyway, so it was no big deal, and I don't think it would be an issue for lighter people at all.
As for the firmness, I simply ordered a Spa Sensations 2" plush memory foam topper for $75 after tax, which made the bed PERFECT! I got a waterproof mattress pad to protect it from sweat or spills since foam seems like it would be hard to clean. I noticed some reviews claimed this had an effect on the comfort of the mattress. This may or may not be true, but I have only slept on it with the pad in place so I don't have a comparison. If you're worried about it, just put it on before you sleep on it at all like I did, and you won't know the difference anyway!
I haven't had any problems at all with sheets or the mattress pad fitting or anything like that, and I honestly love my new bed!
I do have to admit that some of the memory foam mattresses I've tested in stores were more comfortable than this one, but I don't think the difference is enough that I would pay those high prices for one even if I did have that kind of cash to toss around. The difference just isn't significant enough to justify it.