I've been a fairly loyal Enterprise customer (especially when we go to Vegas) for years now. Occasionally I will stumble across a "special deal" from another rental company, try them out, and after thorough disappointment will always return to Enterprise. The service is always quick, friendly, and never pushy. The cars are usually better than what I'm expecting.
Recently I reserved a car through Enterprise, expecting to arrive in Vegas by 4 PM, but our flight was delayed over 7 hours. We got into McCarran very late and were in no mood for a hassle. Unfortunately, the computer system at Enterprise was down at the time (almost midnight on a Saturday night), which didn't help our mood but everyone was very understanding...long story short, we got an upgrade and the rental ended up being free. Enterprise made a potentially disastrous vacation into a totally rewarding trip.