for singlespeed, where chain tension is not as crucial, as I said, I think it should be fine.
for fixies, however, judging from the anecdotal evidence alone (search the SS/FG forum for biopace), including my experience, I would definitely say that biopace does not do a very good job of maintaining even tension. Apparently, it works better the larger you go ( chainrings 50t +).
In a cross application, with a 40-odd ring, it would suck to run it fixed. If you try it, remember also to flip the ring if you use an inner ring on the outer position. This preserves the proper orientation, so they say (I just read it in FG/SS, so I have no idea if it's true).
With the flexibility of a steel frame, given a certain combination of pedal stroke and rough ground conditions, I would habitually throw my chain with 42 bp/15. Chewed up the stays real good.
I'd say if you're curious or have a biopace ring already, throw it on there, take a bunch of hard runs, see if you like it, and let us know what you thought. It's not that hard to swap rings. I think real experience will trump bogus keystrokers every time.