We got the cheapest we could find on eBay (only planning on using them the once) and they were fine. Clothes don't weigh less when vacuum packed but you can fit more of them into one case (we had our luggage allowance pretty much up to the .5kg for each case and in some cases a few hundred grams over - we got away with it!).
Everything arrived as vacuum sealed as when we packed it - have no idea where the bags are now though so don't know if they're good for more than one trip.
EoI in: 5th June 2008 (150 pts, no job offer)
ITA rec: 7th August 2008
ITA back: 30th November 2008
Job offer: 10th January 2009
Arrived: 19th May 2009
Loving it: all the time!