I've been using MT's grid service for a about 2 years. About a year ago my websites started being unavailable for brief periods of time (<30 minutes) at increasing frequency. In november I moved away from them, at that time I had on average one such mini-outage per week.
I've actually heard the opposite. I was planning on moving from BlueHost to Media Temple Grid Server. I know neither are top-of-the-line-or-even-close services. But I don't have any 24/7 mission critical projects either.
We use MT at work and have had dozens out outages in the past 6 months. Your mileage may vary.
Do make sure you leave Bluehost. They denied some stuff about a big malware attack and generally acted ridiculous: