We've used space bags and packing cubes. Since we've started trying to go with carry-ons only for our trips with flights, we've started using the space bags more - managed to do 2.5 weeks in Hawaii this summer with just a carry-on and backpack for each of us!
Though we did buy an extra suitcase at Costco for the Aulani souvenirs on the way home!
Packing cubes are great for summer stuff, and work wonderfully if you roll your clothes. However, we found that even as tight as they get, they can't give us as much space in the bags as the space bags. So if we only need a weekend's worth of clothes, the cubes are fine - but packing for a longer trip, we use bags or a mixture of the two.
That said - if we're checking luggage, we make sure to weigh it as the space bags can certainly compress things to the point that it makes it easy to go over weight!
We do have occasional issues with bags gaining air - but usually it's because the zip-top wasn't sealed properly. Resealling and rolling usually does the trick.
One way cubes are nicer is for unpacking at your destination - you can just throw the cubes in a drawer and be done! Since they unzip, it's easy to leave your clothes in them to keep them organized. You can't really do that with space bags.
We're driving to WDW in November and so won't be worrying so much about space - we'll probably use cubes if anything. However, next summer we're doing a 12N Baltic cruise, and plan to pack carry-on only even with the need for formal/dinner wear, etc. We'll definitely use space bags! (And probably do laundry...