Least favorite place in South America
I was mugged within my first hour of being in Lima, so Lima and I got off to a rough start. (Of 5 friends I know who have recently been to Lima, 4 were mugged there...basically, be prepared for that to happen if you go.) It's a smoggy, dirty, and legitimately dangerous city for tourists. The food is great, there are nice places to visit, and not ALL the residents are thieves and crooks, but there are so many other more interesting and less awful places elsewhere in Peru and South America.
Insider Tip:BE CAREFUL. Arrange taxis in advance through your hostel and agree on prices beforehand. Don't walk around carrying anything you don't want to lose. Keep valuables well-hidden at all times, and use your money belt. Stick to safe neighborhoods, don't expect places to be safe just because they are well-lit or it's mid-day.