I have heard mixed reaction to Renaissance, though nothing extremely negative. The worst criticisms seem to be one of fewer choices, as one might expect on a smaller vessel, and cuisine that fell short of the "fine dining" that one expects on a premium cruise. On the other hand, Renaissance does offer very attractive prices for couples and some people prefer the more casual environment of their smaller vessels. You should be aware that Renaissance's packages generally include two or three days in hotels at the port of embarkation and the port of debarkation, which is great if you want to see the attractions of both port cities but less than ideal if you want an "unpack once, repack once" vacation.
Celebrity has an excellent reputation in the "premium" segment of the market -- which probably is where you get the best deal overall, with considerably more polish than a "mainstream" (or "mass market") line and only marginally higher cost. I would expect MV Millennium to be a class act.
You might also want to investigate the offerings of Princess Cruises (http://www.princess.com
), which also competes directly with Celebrity Cruises in the "premium" segment of the market. The brand new MV Golden Princess, a sister ship to MV Grand Princess, offers the "Grand Mediterranean" itinerary for her inaugural season, which will begin in May (see my reply to Paul's post about the CruiseMates cruise aboard this ship on the Europe message board for information on the ports of call), and MV Royal Princess will offer three different itineriaries in the Mediterranean in May.
Do have a great cruise, whichever you decide!