My thoughts exactly, Rob, about the cost.
If you go to the Yahoo Spiderbeam group, there are guys who are homebrewing them from fiberglass fishing rods. I happen to have four 20' rods from other projects which would be too flimsy at the ends to build a replica of the commercial beam, but they'd be OK to build G4ZU's original design, which for a monoband 20m version would be only 24' front to back rather than 34' (or whatever the Spiderbeam is).
For a monoband beam though, W4RNL's analysis of it shows it to be only slightly ahead of a normal 2 element yagi, though it would be lighter. As you may have concluded from your antenna projects though, a 2 element yagi is a much better antenna than most would give it credit for. I had never used one until I built one a couple of years ago (OK, it was 2 elements on 20, 2 on 10 with a common feed, beta match) and even only up 33 feet it surprised me.
Still, the G4ZU design is interesting. I'm not quite Jones'n for it enough to build it just yet, especially since that fellow was good enough the send me the Giza beam article I hadn't seen since 1983, but I'm still mulling it over.
I'd be interested in a longer-term evaulation of the Spiderbeam too, preferably from someone who's used beam antennas long enough to know what they should be able to do. The eham.net reviews are good, but how many people are going to spend that kind of cash on an antenna and then say it's not good? Comparison to a monoband yagi wouldn't be fair, but to a trap tri-bander would be apples-to-apples, I think.
The most interesting thing to me about that antenna is the balun and multiband parallel feed. I had to read through it a few times to grasp just what they were doing with that (the parallel feedlines from the balun and the heatshrink), but it makes sense. It's just a happy accident that those bent elements raise the radiation resistance close to 50 ohms.
Rick