I'd rather play with a wheel, but F1 2011 is the first real racing game I've played and I'm not sure it's really worth it for one game, especially as I don't know how much longer I am going to be playing it for (depends on bugfixes I guess). At the moment playing with the keyboard puts me slightly faster than the AI on Pro (ABS/TCS/full sim/50% races). Pretty hard without ABS/TCS I guess, though I've not actually tried it.
One thing to bear in mind is that one can simulate, for example, half turn-in by feathering the key. If I tap the key very fast it creates a similar effect. I can take most corners on most tracks pretty much the same way every lap, though I'll occasionally mess it up and run wide. I think fast corners are more tricky because you can't do nearly as fine grained control through them (though you can be finer than people would think), but slow corners really aren't a problem.
Anyway, it is certainly NOT impossible to play with a keyboard. I imagine most PC players of the game are doing exactly this. Perspective on the boards may be otherwise, but it seems to me those who have invested money in a wheel are much more likely to invest in time in the boards...