I've spoken on the show about durability of flash memory devices several times. Because I'm often questioned by listeners and others on this subject, I decided to write a bit about it for you and sort of put it all in one place.
Note throughout that this is just my opinion and based on my experiences and those of a few people I know. This base is fairly limited and your experiences may vary quite a bit. I do walk around with a minimum of 4 at all times, carrying Windows software, Linux distros and all sorts of tools for working on systems, not to mention how-to stuff and my own documents. I use them a lot at home, as well.
Still, I have gone through a number of these things now and I think I can state rather fairly that there are some problems that can affect them.
First and foremost is heat. These devices get warm. The longer they stay warm, the more prone they are to failure, in my experience. Not total failure. They will simply lose capacity. I assume a few transistors die and the system drops that entire block from the file allocation table. Neither I nor anyone I know has had one simply die. They just lose capacity, either in small or large increments, as they age.
Because simply leaving the memory sticks plugged in to the system keeps them quite warm, I have taken to physically removing them whenever I am not either actively reading or writing from them. (Just ejecting them or unmounting them from the file system is not enough. They will still heat up if left connected.) Since implementing that little policy, I have not had one lose even a tiny bit of capacity. Obviously, I do not recommend someone plugging flash memory devices in to take advantage of Microsoft Vista's "Ready Boost" feature. You can do it, but be warned that the boost in performance is not large at all and you're going to have to replace the flash device fairly often. And if you insist in giving this a shot, there are devices specifically intended to fulfill this purpose. Buy one of those. The price premium is because they are designed to take the heat.
And you've already figured out not to leave them in the car, right? That will kill them faster than a 3 year old with a hammer.
The second enemy of memory sticks is dust. It gets into the sockets and kills connectivity. I assume what happens is that the dust is sometimes conductive and crosses connectors, but I am not sure of this. I never buy a stick that folds into it's cover. I want one with a proper cap that fits rather tightly. That way it won't collect all the different dusts and particles that lurk in my pockets and in the bottom of my toolbox and purse. (Yes, guys. I carry a purse. I simply have too much junk to handle when I move away from my home to get along without one. I just make sure it's neither pink nor patent leather and no on says anything about it.)
The third area of concern as far as reliability goes is rough handling. This is kind of a "no brainer", but even the toughest sticks I have seen are prone to damage if they are crushed. It sounds simplistic, but when we chuck these things into a bag or whatever, if we pay just a bit of heed to storing them they will not be damaged. Personally, I just put them into a side pocket and go. Back pockets are a no-no.
I've had experience now with all sorts of brands and models. I have a few favorites. Tied for first place are Memorex and PNY brands. They seem to last the best and offer the best read and write speeds, regardless of whether you buy the fast stuff or just their run of the mill. Running relatively close behind them is San Disk and their Cruzer lines especially.
A word about PNY brand. In the early days of flash memory sticks, their stuff was average at the best. The ones they made early on were in sizes of a few megabytes, and that stuff failed repeatedly on me. Since then, and especially in the last 18 months, they have really brought up their game. I hear this from everyone and my experiences jibe with theirs. I used to avoid PNY like the pest because they were so unreliable, but now will spend my own money on them and gladly so. Even at that, I do steer clear of the older, smaller PNY units; those 256 MB or smaller. I have some and I use them, but only where a quick backup or replacement is at hand. I have learned not to trust them the hard way and they never leave the workbench. It is at the least embarassing to have one fail in front of a client.
I am not aware of a "bad" brand; one that you need to steer clear of.