Sigma has two lenses that match up with each other pretty well, I'm just not too sure on their quality and how well they'll play with the A55;
1) Sigma 18-50mm F2.8 EX DC Macro
2) Sigma 50-150mm f/2.8 II EX DC HSM
Two lenses covering 18 to 150mm in total with f/2.8 all the way through. Both have reviews that say they're soft on the 2.8 side but are crisp starting at around f/4 and up. Mixed reviews on their autofocus ability, some say it's quick and quiet and one person said it hunts in the dark while another said it's 100x better than any kit lens.
There are a ton of top level lenses out there but I keep thinking that I'll be using a ~$700 body and I should find a lens that matches that well, nothing too much more yet definitely nothing less.
Honestly the only thing that scares me about these two zoom lenses is keeping them clean. I remember from a long time ago when I had a SLR and zoom lenses they would suck dust into them and I couldn't clean them.