YMMV with the Splenda -- to me it tastes of swimming-pool. But in general, yes, I have found that when I give up sugar (which I do periodically and need to do again) it takes me a few weeks for my system to calibrate. But I don't get cravings for it after the first week, which makes it easier.

Stevia powder's great stuff, incidentally, if you're looking for a non-sugar, noncaloric sweetener. And not weirdly chemically engineered a la Splenda, if that matters to you.