Using fizz keepers, pump bottle tops for bottled beers ?

I currently bottle my brew into plastic 2 litre fizzy water bottles. And they are fine and so far have no trouble holding the carbonation. They are also really handy for testing when they are fully carbonated.

The thing is on the occasional night where I dont want all 2 litres by the next day they are pretty flat.

Has anyone tried these things :-
http://cgi.ebay.co.uk/ws/eBayISAPI.dll?ViewItem&item=230253837993

Either for beer or other fizzy drinks.

Obviously they just pump air in so the beer wouldn't last that long but for overnight do you think it would be ok ?

pumping normal air into a bottle will oxidize the beer. If you drink it the next day it might not taste stale. but 2-3 days would be the maximum.