I've done a few, back in the early days, and they do seem to go a bit 'flat' after a few months, though the bottles are still pretty hard when 'tested' by squeezing. I personally have decided not to put anything in plastic for long-term, but it's fine for short term stuff I expect to drink reasonably soon.
For reference, I'd bottled some beer last May to leave at my BiLs' house (they're serious about no glass at pool, despite the absence of kids), and the few that survived the summer were a bit flat come December--so 6 months is a longer than my bottling skills and plastic are capable of. You may fare better.