What do you mean by "not good"?
The thing is, it's much better to have an even number in memory's capacity (2GB, 4GB, 8GB - and so on and on), than to have odd number (1GB, 3GB, 5GB and etc), because it can cause problems with memory addressing in some OSes and cause graphical glitches or errors.
It's a rarely happening thing with modern video cards, but it's still better to use even-numbered memory even today, just to be secure.
And since I personally always prefer stability and longevity to anything else, I don't like "2GB+1GB" or "1GB+4GB" CFX/SLI setups. It's my personal preference, it doesn't mean you have to refuse of such setups just because I said I don't like them. Majority of the time they'll work just fine, without any problems at all.