Yes, but you have to make sure your Vram chips are cooled as well,
Plus you can swap your cards without losing the covers, also some cards don't have F/c blocks available, non reference edition cards for example
Grab a uni and some ramsinks, good to go :-)
Vregs and Vram needs heatsinks, like Moto mentioned. Swiftech has some neat universal blocks with full cover card heatsinks you can use with their MCW60/80/82 series GPU blocks, but I think only for reference PCB cards. I've saved a lot of money over the years by only spending ~$100 for the 2 blocks, vs $100+ per block, per card.
well I have the MWC60's, 4 of them in fact, and I use them for all sorts of stuff including GPU cards. but I have found that in some cases the full cover blocks can be slightly modified to fit newer cards. like the 6800 full cover can fit a 8800 GT by trimming off a little on the pointy end so not to hit components on the board. but I look for design when buying parts, not just performance. I have a 8500 full cover block on a 8800 ultra, a gene I had to trim off the copper add on section designed for a ram cooler to be bolted on to it. but this is not for everyone, and if you do not have the tools for this, than do not try it. this is not easy and it is easy to mess up a block. so I guess what I am saying is it's up to you to decide what you are willing to do to make things work. I jest bought a GTX 480, and I plan on trying to make that 8500 full cover fit it. I may fail but not before trying.
Milling or modding a full cover block can cause serious issues if you aren't careful- either by leaving some component uncooled, or by having a piece of the block actually short out the card by making contact with a PCB component that it shouldn't. I know several people have modded 'similar design' blocks for card layouts, but this is very much 'do at your own risk' task, not to mention you also void the warranty of the block if it ever needs replaced due to manufacturer flaw.
I switched from full covers on my 6950s to unis on my 560Ti, and I don't notice a significant difference in temps. Both types of blocks do well cooling the GPU, which is the major source of heat in both cases.
I'm a frequent upgrader, so having the uni blocks is a huge benefit. It took me almost 9 months or so to sell off both GPUs and both waterblocks since they're model specific (selling the GPUs took only a few weeks with the stock coolers). If you don't upgrade as often, this may not big as big an issue, but selling GPUs with full cover blocks literally is a tough sell.