My DS 8, who is very mathy, is finishing AoPS prealgebra 1 this week. He loved it. Even though the classes are all typed, they are engaging. DS also really enjoyed the online videos, he found them entertaining. He thought the pace was a bit fast, maybe because this was the first time he has ever had to put any effort into a math class.
He is going to try the Elements of Mathematic now, as prealgebra 2 does not start until Feb-Mar. He went last year to IMACS for a math enrichment class and really enjoyed it. The classes at IMACS are not meant to be a stand alone class, but rather enrichment. I am not sure if the Elements of Mathematics works the same or not.
Petunia: There is only level 1 right now for Elements of Mathematics. On their FAQ they state you have to start at the 1st level, but more advanced kids will be able to work more quickly through the levels.
