I recently bought the sigma 17-70 along with the canon 550d. It looks to me not sharp, not crisp, looks worst like the older 350 with the normal 18-55 canon. Is this normal for sigma?

If you bought these locally, this would be a matter to take up starting with the store where you purchased the items. This would be the quickest path to diagnosis presuming they would have another lens or camera for comparison. If bought on-line, contact the seller and describe the problem. In either case, they may elect to replace or repair. You could also contact SigmaUSA repair and ask for an RMA authorization for inspection/repair. Since there are two equipment brands, finger pointing is a possibility.