We've got one and I'd hate to be without it.  It's outdoors though, so pit fumes aren't much of a problem unless a diesel truck is running with an underdeck exhaust.  Never had any gasoline fumes ignite.  Usually the biggest problem is too much ventilation from a stiff breeze.  Spend quite a bit of time walking to the garage for the right wrench/socket though.  Also, it was plumbed for air but the underground line broke.  Either corrosion killed the copper or it filled with condensation and froze.

  The only trouble with it being outdoors is that it's a drunk collector.  Had one drunk and stoned fellow fall into it one night.  No major damage, but I'm glad he was whacked when he did it.  Hate to have a sober person hurt themselves.  It also collected a drunk neighbour who tried to park his car over it and missed.  We usually park a vehicle over it when it's not in use now.  Either that or we put up posts with ropes to keep the drunks from falling in.  Stories like that make me think that a half-pit is a good idea.