While I recently bought the Marantz PMD660, a friend bought the Edirol R1. He actually ordered his months ago when it was first announced, and it then took months to be delivered. I don't know when the PMD660 was announced, but by happenstance we both took delivery of these solid state recorders within about two weeks of each other.
They are somewhat similar in functionality and application, yet they are also very different. The R1 is much smaller (though not as small as my Sharp mini-disc recorder) and records at 16 or 24 bit, while the PMD660 only records at 16 bit. On the other hand, the construction of the Marantz appears to be a little more solid, it has XLR connections (and phantom power) and separate level meters.
Both also have built-in microphones; that's right, two for stereo recording, though given the mics are only two or three inches apart, you aren't going to get great stereo separation!
My friend used the Edirol to record a recent concert using the built-in mics and was impressed with the quality of the recording. It certainly didn't seem too bad given the situation.
I think the big question is how important the 24bit recording is to you. If it wasn't for that option, I don't think I'd consider the R1.