Thanks for the link doug, I thought everyone had forgotten about that how-to (ahem, yo Chris, you ever get that posted in the tech section? Just rib'n ya bro).
The first set I did on my Savage lasted a couple gallons of hard core bashing. I replaced them because of wear and dry rot due to UV, though they did have some life left in them anyway.
I almost got a set of the lycra ones at the LHS this past weekend, but I didn't like the looseness of them on the top of the shocks. Personal preferance I guess, but I stuck with the shock condoms. Whatever you use, they will pay off. On the set I had on the Savage, there was nearly no dirt contamination in the oil, and thus the seals were not worn or loaded with oily dirt. This is the most important factor in using shock covers, not having to clean the shocks is just a bonus, not having dirt destroy a shock is the main thing.