I discovered those vacuum stoppers a while ago, and now use them for all sorts of wines. As the others have told you, there's some fall-off in quality with a normal wine. It seems to me the most noticeable decline happens between the day a bottle is first uncorked and the next time it is used. Thereafter, the decline seems to be smaller, or at least I notice it less. The drops in quality seem to be based on the number of times the bottle is re-opened, not so much the number of days it is kept. I have opened a bottle, vacuum sealed it, and then gone on a business trip for a few days, and when I got back, the wine seemed to taste like it normally does on the next day.
For really good wines, I wouldn't use the vacuum stopper; like Suzanne said, pouring into a smaller bottle and stoppering it without an airspace is the best way to keep it. (Catalog outfits like "Wine Enthusiast" and "Wine and All That Jazz" sell a set of carafes with stoppers for that purpose if you don't have some already.)
Fortified wines, like sherry, marsala, and port don't seem to degrade nearly as much with a vacuum stopper as normal wines. I keep bottles of those around for cooking and vacuum stopper them after each use, and they remain usable for weeks - sometimes literally weeks; I do a lot of cooking for one, so don't use a lot of anything at one time, and full size bottles of cooking wines can linger on for a long time. For example, I think the sherry in the cupboard right now is a bottle I bought at the end of February. I sampled it last night since I was using it for a stir fry (they say never cook with a wine you wouldn't drink, right?
), and while I could tell a difference between a newly opened bottle and that one, that had been opened and re-stoppered a few times, the difference wasn't objectionable.