Really depends on budget + goal.
I've personally had 2 dual GPU setups (3dFX & current 4870s), my brother has one (6950s 2GB) as well, a few guys from work too (560TIs & 4890s) and aside from very minor glitches where all you needed to do was turn off SLI/CFX until driver was fixed, they've been awesome.
My only real recommendation is never settle for cheap/weak setups, because IF there are any problems, you're stuck with a weak single card until everything is fixed. The setups I mentioned are based on cards that are still pretty strong in a single configuration (ok, my 4870s are definitely getting old, but even a single one gets the job done if temporary).
Like supernova said, to anyone wanting SLI/CFX, much better idea to go single strong card for a little while and get the same one when budget permits than go right away with 2 weak cards.