I've rented tuxes in the past but as a frequent cruiser, I found that buying one would pay for itself in a couple of cruises. You can get them more reasonably priced than you might imagine, especially when there is a sale. Having a tux, I've used it for other ocassions on shore, like a New Year's Eve party.
As far as the ladies, it is quite easy to pick up a top that goes nice with a long dark skirt that will do the trick. I'm sure the ladies here have lots of suggestions on mixing and matching. My wife picks up those on sale too.
I purchased a used tux from a tux rental company. They have all the accessories and they only use the best of product. The price was right and it saves renting every cruise. The only problem you could have would be eating too much on the cruises and expanding the girth somewhat.
I have used Cruiselineformal.com for the last 2 cruises on Princess. Works out great for me. One less suit to pack, 2 less shirts and not ties. Leaves more room for Scuba gear.
Seriously, the quality you get is very good and provided you take good measurements, the fit should be good too. I have rented the White Dinner Jacket and black pants. Comes with 2 shirts,tie and cumber thing.
guys in our group have rented tuxes using the company that Princess recommends, the tuxes are classic tailored & cut - sometimes designer. They look very nice.
The ladies formal wear looks very nice also - usually a long black skirt with a very dressy blouse.