Doesn't Chuck Norris promote something like that? If he uses something like that then you know its gotta be good!

They say when Chuck Norris does pushups, he doesnt push himself up, rather he pushes the world down.

It wouldnt let me post the photo as an attachment on my first post...here is Chucks TP

I do have some experience with these as well from my swimming days.

I think the benefits are more for the dedicated pool swimmer rather than the triathlete, though you may find some of these uses benefitial at times:

1) It can replicate the feeling and stroke of freestyle, and you can put additional weight (raise the angle), to do stroke based strength training.
2) If you put it up against a wall, you can lay on your back and do pushoffs to strengthen your flip turn, obviously not really necessary for triathletes.
3) If you have an ear infection, you can use it to maintain feel and strength out of the water.
4) I would honestly reccomend just gettting stretch cordz, you attach them to a post and you can strengthen your stroke that way, can be done vertically (hanging overhead), or in front (you bend over while standing): http://www.swimoutlet.com/product_p/1225.htm

We used them back in HS for dry land training to supplement work outs. They actually are helpful tools, but that was when I was putting 5 hours a day into swimming and was concerned about it. Again though not in replacement for a work out but rather as a supplement to it.

seems to me like a bungee cord and a bench would do the same thing.