I've been listening to the Lost Tapes regularly for the past two weeks. It is a definite purchase. No, it is not as good as their classic albums, but it is still pretty great. Better than what we could resonably expect, in fact. The studio tracks all range from the very interesting to the great. Based on the studio tracks, I'd rate "The Lost Tapes" as the equal of "Unlimited Edition."
My only complaint is that I find the inclusion of live tracks a bit unnecessary. The live tracks are fine by themselves, but I don't like them interspersed with the studio tracks, and frequently skip them for later. I would have rather the set be a 2-CD release of just the studio tracks, or maybe keep all the live tracks on disc 3. That's probably just my listening style, though.
Bottom line, it is an essential purchase if you like Can!