Miked.
That's really the only one.
Actually, I started with the BL demo in first half 2007, left because I couldn't get the $ to buy full, and joined Roblox in June 2008.
Then, the community there crashed, and I came back. With the full version. I still play Roblox because it's more achievement-based, and Blockland is just a whole lot of building. And taking forever to join games. But Roblox is glitchy as hell and Blockland has nice particle effects, which I would like to see incorporated into Roblox.
Which will probably introduce more glitches (because it always does, they just put in new stuff (and new glitches) to get more players) -- Roblox is becoming really stupid about promoting their "free" game whenever possible then trying to get you to pay by spamming you.