Prices of 3dfx cards are becoming insane, are they worth it? You can ask that question for ANY old part and only you can aswer that question....
I think they're still worth it, but you need to be smart about it. If I didn't already have my Voodoo 2 and 5 cards, I wouldn't be buying them now. To me, the price is way too high.
However, I still think that building a real hardware GLIDE machine is worth doing and can be accomplished at a reasonable cost:
440BX Motherboard $40
P3 650mhz Coppermine $10
256mb PC100 RAM $10
Voodoo 3 3000 AGP $40
Voodoo 1 PCI $40
AWE64 Value $20
HDD & DVD/CDROM $25
PC Case $Free (craigslist)
So, for ~$185US you can build a rather kickass GLIDE machine. When you want to run the Voodoo 1 card, just underclock the CPU to 429mhz at 66mhz FSB. Now you have full 100% GLIDE compatibility for not too much cash. I think this is still reasonable.
If you want even more power, find an i815 motherboard that allows you to set FSB at 66mhz, 100mhz, and 133mhz. Make sure you can also set AGP and PCI dividers. Then, get yourself a P3 Coppermine 866mhz CPU. You just need to find the right motherboard to make it all work.
I estimated these costs on the high end. You can still do this for much cheaper if you diligently search craigslist for freebies and are extremely patient.