I split at a Vault firm in DC and a mid sized lit firm in Dallas last summer, and had very different experiences at each.
At the large firm, I did almost no substantive work of any kind, a few memos here and there, some brief checking and cite checking - I think I drafted portions of two different briefs that would never really see the light of day. I spent most of my time researching and summarizing the results of my research in memos. I was taken to lunch almost daily at great restaurants and we had multiple social events every week, from events out at bars to summer league softball, the whole nine yards. And when you factor in the money, it was amazing. But none of it was why I wanted to be a lawyer, and I think a lot of people get lost in that. I want to do trial work, and I met with people there who had been working for years and never saw the inside of a courtroom. It felt a lot like I was working on little things that would get put into slightly larger things that would one day make up even bigger things and then way down the line someone else was actually doing the lawyering. I know that it was just being a summer associate, but I didn't see fifth and sixth year associates doing the kind of work that was interesting and meaningful either. There was a serious strata between partners and associates, and I never got any substantive feedback on anything I did. After it was all over, I felt like it was an awesome summer vacation, but other than the great pay and the things I could do with that, my legal skills weren't going to improve very fast there and I wasn't going to really enjoy the work.
On the other hand, when I went to Dallas, it was a different ball game. To give you some idea, the firm I was working at is about 50 attorneys, so not a large firm, but not a little mom and pop shop, either. First off, the climate was very different. All work was substantive, and I was treated essentially as if I was a first year associate. I had work from associates all the way up to the managing partner of the firm, and when I was asked for my opinion on topics, it was actually taken as a legit opinion. I got regular feedback when I asked about it, and they didn't pull any punches when I fucked up, but also didn't give me shit for it, either. In one situation I was asked to do some research on a topic, and when I came in to give my results to the managing partner, he asked what I thought we should do next. I told him we should file Brief XX, and he paused for a second, then asked if I felt comfortable writing it, and so I did. I knew that later on it would get modified if we ever needed to file it, but it felt like at least my work was worthwhile. On top of that, the Dallas firm actually goes to trial on a regular basis, which is exactly what I want to do.
After working for both places, I knew I didn't want to work at Biglaw. There was a 25% pay difference between the two, but in the end it was an easy decision. I also worked for a small firm of about 6 attorneys and got the feel for that as well, and now that I've been in each environment, I took the job at the Dallas firm. I think the number one thing for people to realize is that you need to think about whether you will be happy at a place long term before accepting the job, if you do have the option. If your only concern is how to get the offer, then just bust your ass on your work as best you can, kiss ass (if you are actually trying to get a feel for whether you are a good fit for the firm, then be yourself, but if you just want the job, just make them like you and tell them their shit don't stink and you love them to death - most people in law have healthy egos), and don't drink too much or make a fool of yourself in any way at social functions.
The only errors I've ever heard of at most firms (especially the ones that offer almost everyone, like a lot of Biglaws) are people who act out in some way - be it by getting drunk, saying strange things, or just not fitting in. Personally I don't recommend that if you have options, but if you just need a job then do what you gotta do. I think that people should realize that the most important thing to being a successful person is being happy in what you do. Before law school I had another career for about a decade and I loved my work, and that was why I was willing to put in 12 hour days. If you can't see yourself putting in 10-12 hour days at a law firm from time to time because you love the work you're doing and it interests you, find a different firm.