I have the 100mm f/2.8 macro lens from Canon and it has been spectacular. It costs a bit more but IMHO is totally worth it and then some. It is sharp as a tack and has performed beautifully under all situations.
If you check my tulip post from yesterday you'll see a good example of it in action. All my macro shots are done with this lens so any macro shots in my archives are good examples of this lens.
Almost all macro lenses are good but on a a camera with an aps-c sensor like the 550D a focal length of around 60 mm is practical because it's more or less the perfect focal length for portraits also. The 100 would be a bit too long for that (unless you have a lot of space). I can recommend the Canon 60 2.8.