The Sheraton is fine - well run by local standards, and safe but very expensive, make sure your per diems are healthy to cover meal costs, etc. There is a small supermarket directly next door that many of the pilots use. There is a good social life within the hotel as it's usually buzzing with crews, but it can feel a bit confining. I wouldn't stay in any other hotel in Lagos. Apart from the odd power cut and the gennies kicking in, the crappy choice of stations on the TVs in the rooms, or the broadband dying (only occasional), it's liveable. Not one of the guys I know have had a problem there, and most were or are on 12 month contracts. Many of them were very cautious for the first few months, and then gradually but carefully expanded their horizons.
It really depends which airline you're working for and their level of professionalism outside of that (pm me if you want). You need block leave off and flights home so you can work hard when you're there as there ain't much else to do, and then go home on leave and chill out. I wouldn't wander around, but during daylight with an airline driver who won't dump you in the wrong place, it's fine. Once it gets dark it's a different story, and the better airlines use either their own security or private companies. Port Harcourt and the oil-producing Delta region is a different proposition altogether, and is responsible for most of the nasty stories your family will read about when you're on contract. Now they're past the elections and it didn't kick off down there, it's worth considering.