I have various file guides (Granberg, Stihl, Pferd, Oregon, Husqvarna) and grinders, the little Dremmel tool type and the Dremmel attachment, bench grinders...
They all have some pros and cons but they all do the job. My personal favorite, though, is the
TIMBERLINE one. I have extra cutters and guide bushings for it, for all the chain sizes/types that I use.
Badly rocked chains, or super dull/abused chains my neighbors or clients give me to sharpen, I start with a bench grinder. Those will get them back into shape quickly so you can sharpen them with one of the other devices in the future. Hand filing without a guide takes a lot of practice, the guides will give you better results right off the bat, and have a shorter learning curve. I got much better at touching up a chain with just a file by using the guides for quite awhile. They help you learn to hold the file at the right angles and perpendicular to the bar. They give you time to develop some "muscle memory" doing it, while getting decent results.
The Timberline is my favorite because I take it with me in the field and on jobsites. It can sharpen a rocked chain, and touch up one that's just starting to dull. It comes with a carrying pouch and is small, compact and not fragile.
It works me, but I have a friend who bought one after seeing me use mine, and he ended up trading it to me for a couple of the Stihl file guides. He didn't like it. Never elaborated on why, but he's not a patient guy, so it might just be that it confused him a bit. He's also not exactly famous for reading instructions or manuals. I ended up with a spare/backup Timberline and he actually got real good with the file guides, so it's all good.
I think we all just end up using whichever device feels the most comfortable or rewarding to us. Once you get the hang of it, sharpening chains is rather relaxing and you always get the satisfaction of knowing that the time spent saved you money. You'll be shocked at how many times you can resharpen a chain, especially if you do it regularly...
BEFORE THEY GET TOO DULL!... like others have said. I also have never actually counted how many times you can resharpen one. It varies too much from chain to chain, and how bad they were when you started. Waiting until they look like a dull butter knife, though, means more filing, and sharpening them as soon as they start to feel like they're getting dull will certainly
INCREASE how many times you can sharpen the chain, even though that seems wrong. When they're in bad shape, you have to take off more metal than you would touching them up a half dozen times.