I think they're great, and being about 30 challenges into it I can already tell you personally I'm not going to be able to do them all. Some of them you just have to keep doing it and as mentioned above YouTube helps and it's really not a big deal or "cheating" to see how they're done properly.
It reminds me a lot of various other time trials and whatnot (MGS VR Missions anyone?) so that's what I like about it.
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http://mikedinthehead.wordpress.com