I'm currently using both, and found they both work just fine. Use Nhancer for SLi, use RadeonPro for Crossfire. nVidia gets a +1 because they will let you modify game profiles in the driver, whereas for Crossfire, you need RadeonPro to do it.
As far as "worth it", that's subject to your desires. Both options are great for upgrade paths, or as current high-end machines.
Just think about it - before SLi and Crossfire came along, the only option as an upgrade was to buy a new generation card. With Sli and Crossfire, you can buy a second last generation card and often get very close to or higher then a current gen setup, for less money.
My usual MO is that I buy the top of the line single card for the current generation - then I upgrade when a single card comes out that is faster than my current card in SLi/Crossfire. My previous card was a GTX 280 FTW - my 5970 is supposed to be faster than 2 x GTX 280's in SLi. My card before that was an 8800GTX, the GTX 280 was faster than 2 x 8800GTX's in SLi.
That's just how I roll