If dumbells are your only option, then better than nothing. I'd rather have dumbells than just bars or a smith machine in any case: you can pretty much work any muscle group with them.
Dumbells will get you better range of motion, and work your supporting/balance muscles better than bars. The major drawback to just dumbells is you'll have a tough time with leg work outs, and as stated squatting and deadlifting will be pretty limited in weight (although possible).