It's not how strong it is, it's how you use it! ....
That was a terrible joke and I am sorry.
But, in all honesty, I do believe it is the very point that the power of a devil fruit highly depends on the user.
Ace only became stronger with the Mira Mira, but does that mean any-one eating it would just become as strong as he was?
I don't think so, stronger yes, but as strong as him? not neccesarily, perhaps with training, clearly you can train your devil fruit power to become stronger. We've seen that.
Also the strenght of a devil fruit HIGHLY depends on who you are fighting.
For instance, Smokers ability to just turn into smoke and fill out a room, only manifisting a fist when he needs it to manifist, to punch some-one with.
Is really neat until the moment he is up against a Haki user, and turning into smoke would just become a dumb idea because the haki user can just hit the smoke randomly with his haki, and as an effect course damage to Smoker.
And of course we have Usopp around to prove that you can actually win fights with creativity rather than strenght, and some-times more weird abilities are the ones you need.
Remember how Zoro and Co. were absoutely powerless against Perona?
The only one who could even stand up to her was the none devil fruit user none monster whom didn't even have Haki.
So really, depends on the situation and just who exactly you are up against.
As well as how you decided to train and use that said devil fruit power.