I actually think the 3 formats we have work well.
Test
I love the chess-like twists and turns of the long format, how a game can ebb and flow and what seem like tiny moments at the time can have a huge impact on the game. Test cricket is simply that, a test of your patience, your technique, your mental ability and much more.
One Day
I quite enjoy 50 overs as you can get a decent cricket fix without having to commit to 5 days or it being a boom boom t20 affair. It's great for having a day out with friends/family and still has elements of ebb and flow that I like in test cricket. Do the middle overs drag a bit, probably a fair bit, but then some of the best cricket matches I've ever seen have been low scoring ODI's.
T20
I love the buzz of turning up to a T20 here at Taunton, the atmosphere always seems a bit more charged and it's a perfect way to spend a summers evening with friends having a few ciders and having fun.
I think the length and pace of the 3 formats we have work well and it's quite obvious that the shorter formats are having an influence on both each other and test cricket. If you look at any of those formats from 10-15 years ago they have evolved quite a fair bit. T20 certainly isn't an awful format, it's a great way to introduce people to the game and I've got friends that now watch all forms of cricket simply because they found the commitment to a 3 hour T20 much easier to swallow than a whole day or 5 days of cricket - these people are now into test cricket and love that format yet without the short format they probably wouldn't have got into the game at all.