Shadows Over Camelot - absolutely, we always play w/a traitor. Half the fun is assuming someone must be the traitor - because that was so stupid? - then next person does something even dumber and you're very confused.
Sherlock Consulting (new version, Jack the Ripper) - cases 1-4 are Jack the Ripper and they play sequentially. We haven't started those yet. We've played cases 5, 6, and 7. If there were any errors - we haven't noticed them. You definitely need i'd say at least 4 people though. We always play 6, but I think it would be fine if you have at least 4. Less than that you wouldn't have the creativeness i'd think. You really need to pay attention to the most mundane clues, particularly early on when you get the case description. We play it "live", meaning you can't go back and re-read the initial visit/case description, because in real life you couldn't go back either. If you want to re-read a witness/suspect account, it counts as another clue used.
It's hard to describe w/o giving away info about cases, but all 3 cases we've played we solved, they made sense, and we definitely had to come up with some theories after each clue (fake examples to follow): Maybe he has two families? Oh, i'll bet he has a hook for left hand! He was at the hospital as an employee, not as an accident victim! Stuff like that.
The Sherlock answers are a very minor irritation. Basically Sherlock will hit the correct spot to ask questions every time, so for example, case 4 we solved in 12 clues, which we were very impressed with ourselves. Sherlock had done it in 6. We still scored well and it's just such a different feeling after a game. All the cases we played after dinner, dusk to dark, and embraced the theme. We didn't use bad English accents or anything, but we took on the atmospheric feelings of old London for sure.
Also - for those that play, we use "perler beads" in different colors to mark locations we've been, think we should visit, or were otherwise of note. It's very helpful for establishing likely traffic patterns, or "if he would have had enough time to do both murders", etc.