I think the best ways to get timing down are to practice along with actual songs as well as getting a drum machine.
I know that over the years my timing has improved tremendously since I started using drum machines. I can remember listening to early bands in the 70's and 80's and thinking how great they were. Then listening to the same bands 20 years later and wondering why I never noticed how bad the timing was on some of the songs.

After so many years of using them now, I can instantly tell when the timing is even just a tiny bit off. Even the slight lag caused by excess midi information in software is noticeable now, when I would never have even been aware of it back then.

Another advantage of using a drum machine is that you can program in different styles and beats. With a metronome you're keeping beat to a 4/4 click, but you can't really get into the backbeat or change your strumming pattern that much.

Other alternatives to drum machines would be using some kind of recording software to play midi files. Then you could also practice along with music as well.
Another alternative is to use loops. For instance, the Jam tool on Guitar Tricks is an excellent way to practice timing as well as learning scales etc.