Molly most definitely has a taste. If someone's sold you what they claim to be molly, and it doesn't taste of anything, it's fake. As in throw-that-shit-out-right-now fake.
And it's very rare to find Molly that's been cut. Mainly, this is because of recrystallisation problems (as mentioned earlier). But it is also due to the fact that molly has a very distinctive taste, and most people who take it regularly would be able to discern if it was cut with anything.
This isn't to say molly is always 100% pure MDMA, there will ususally be a few impurities that have made their way into the crystals. But these are few, so you're looking at about 97-98% purity as a worst case scenario.
I'd agree with quite a few people here that it's becoming more prevalent in the UK, especially around London and Universities. At my uni there's a guy on campus who regularly sells it, and freinds at other uni's have little problem getting ahold of it. Where I'm at, pills are harder to come by than Molly.