Well just witnessed a rather decent little 20/20 game between Essex & Sussex. Scores tied with 4 balls to go & Essex (chasing) get a little chokey; lose two wickets in successive balls meaning on the final ball Ronnie Irani needs 4 for his ton, Essex need one for the win & Yasir Arafat is on a hat-trick...
Big Ronnie calmly carves it to th on-side boundary for his ton. Top stuff.
That's not an argument for or against, btw. For me the argument basically boils down to whether you consider overs 15-40 to be worthwhile in ODIs. If you do, you'll prefer them to 20/20s; if not you'll take the shorter form. I'm not suggesting that they can't be totally crucially to the outcome of the game, but if they are it's usually because of what has gone before: be it quick wickets taken in the first few overs leading to the necessity of consolidation; or a chase of a huge total (like the Oz-SA run-fest mentioned earlier) meaning the foot has to remain on the pedal.
People criticise 20/20s as a batsman's game, it unquestionanbly is. So too are all one-dayers tho; as SJS correctly points out you don't (strictly speaking) have to take a solitary wicket to win one. However, the art of "bowling at the death" is recognised as a skill in ODs, what 20/20 does is effectively ask bowlers to spend their whole spell "at the death". Bowlers who go for fewer than six per over
at the death in conventional OD stuff are rightly praised for it, to me it just seems churlish to criticise praise for economical 20/20 bowlers when they're effectively performing the same skill.
In the game I just watched Michael Yardy (part-time SLA) bowled a very decent spell of 4-0-21-0. Now if one thinks of that performance in conventional OD terms it doesn't look very flash at all, but if (say) Glenn McGrath sent down 4 overs at a bit over 5 per over with a team chasing over 9 per over I think he'd be (rightly) praised for it.
Ultimately it's purely personal choice, one format isn't inherently better than the other, but it seems to me that a lot of ODI fans are praising the format because it more closely resembles first-class cricket (no argument, it does, albeit not very much at all just more so than 20/20s), but that seems to be missing the point of the shortened game. 20/20s strengths & weaknesses are at least its own.