I've been using them for my batches so far. With my last batch bottled, I also kegged 3 gallons, so only a few of the Grolsch bottles were filled (also did a 6 pack of 12oz bottles). I've had some duds, but I attribute that to being a new brewer. My first two batches in them were 100% carbonated.
IME, You do need to line up the gaskets with the opening and make sure they seat properly. This only takes a second and both hands to do. I've found that if I do that, I get good carbonation. If I don't line it up right, or the gasket has other issues, you might not get a good enough seal. I've started to flip them over after each batch, to ensure good contact with the next batch. This method is working out very well.
Since I'm getting into kegging, I don't expect to need to get new bottles. I do have a 100 pack of fresh gaskets, so I should be set for some time to come. I'm also using Belgian bottles, corking them (and using the cages)... Those bottles have a 100% success rate.
I will say that there's something really rewarding when you open up either a Belgian or Grolsch bottle, hearing it go POP! You don't get that with 'pop' tops. I opened up a bottle just a short time ago (tonight) and it did a nice, loud, POP when I opened it. Very nice carbonation, thick head that held up for more than long enough. There's about an inch left in the glass (of brew) and it still has about a 1/8" thick head on it. :rockin:
If your swing-top/flip-top bottles have had more than a couple of batches go through them, inspect the gaskets. If they show deep marks on the gaskets from the bottle top, either flip them over or change them. I would plan to get 5-6 batches out of the gasket on the bottle. So you might want to keep track of how many have gone into the bottles. I would still flip the gasket over, depending on how it's wearing. Of course, it wouldn't hurt to flip it after each batch, just to be sure it makes solid contact with the lip.