I've been using my Giant Halfway on my work commute. I'm about 6' tall and I think the bike fits me OK. There are times when I wish there were more handlebar positions though... and that's something I'll look into with bar ends or a new handlebar. The seat tube can extend up beyond my maximum needs so it should theoretically fit someone as tall as 6'4" or so. Someone taller could add a "Butt Buddy" or a sprung saddle for a little added height.
The gears work well, though I'm almost always on "7". Which means that the only way to go faster is to peddle faster... no more gears beyond that. Someone on these forums suggested changing the gears to an 11-28 freewheel (which is no longer available from Shimano, though there are rumors of its imminent return)... and that would make the 7th speed a little faster... which would be nice for me. The low gears are great for hills, but I'm in the midwest and the worst I come across is decidedly not a hill. LOL
That said, my grip shifter is starting to show signs of wear and tear. I'll be fixing that this weekend.
When folded, the bike doesn't have a way to stay together on its own. I use a bungie cord to strap around the saddle and the handlebar post. This keeps the bike together for carrying and allows the wheels to still move. This is important as I generally don't fold the handlebars down. Instead I flip the bike over with the handlebars up and I roll it along. Makes carrying it on and off the train a lot easier.
I have had some minor issues with the folding latch. It's a very elegant design, but you want to make sure that the bolts and whatnot are checked out regularly. It's a bit tough to work with them. I use a needlenose pliers and an allen key.
Some folks here have complained about the process to take the tires off. I found it to be a lot easier than I expected. Though it's not necessarily something I would want to do on the side of the road with cars buzzing past.
The bike is heavy. At 28+ pounds it's among the heaviest quality folders around. I took off the rear mud guard and rear rack to shed some pounds, but I'm still at 26.5. That doesn't sound like a lot, but try a full day of multi-modal commuting where you're getting on and off trains and hefting the thing up and down stairs. If, however, it's not being used for this purpose... then weight isn't an issue.
The ride is good and as smooth as your road surface. If you're looking for true comfort though you'll need to invest in fatter tires (Schwalbe Big Apples have been suggested to me) and possibly some sort of seatpost suspension. I don't have either and I feel ever bump in the road. Not that it's a bad thing necessarily... just depends on what you want from the ride. These things are so subjective.
Now... for the big question. Would I buy it again? No... as it's not well suited for my purposes. But then again this is my first folding bike and I didn't really know what my purposes would end up being at the time. I kind of originally bought it to keep in the car for a quick ride here and there. For that, it was great. For my current use, however, I'm saving up my pennies for something else.
It sounds like for your purposes (riding to the corner store and back), the Giant Halfway will work well. That said, keep your eyes open for other folding bikes and give them a test ride.
--sam