I'm looking to get some zoom lenses for my Rebel XSi, and after some research I'm still confused... So I'll ask the UCP professionals.

I almost bought a Canon 55-250mm lens, retails at about $300, can get it on ebay for around $230. I've tried this one before and know that it works fine, but then I saw a Sigma 70-300mm f/4-5.6 DG Macro Lens for le$$. Are Sigmas any good? is the 70-300mm a good zoom?