Well, it took 10 years for my HP 4L to have any issues. But, I've seen some issues with 5 and 6L's. Usually, the problem isn't with the actual printing but in the paper feeding. If the printer is functional, the quality of the printing should still be Excellent. 600dpi on the 5L and later. The speed is slow by today's standard, but, certainly plenty fast for your average use.
My Dad has a 6L, which shares the same form as the 5L. The problem that has developed for him is that the paper feed is messed up. It doesn't draw in a single sheet at a time, but has the tendance to pull in 3 or 4 at once causeing paper jams. We need to babysit the printer when we use it to prevent it. I will mention that he RARELY uses it. I think he prints 20ish pages a YEAR, so, I think that has something to do with it.
If it's a working printer, It's definately worth using. Especially if it doesn't have the paper feed problem my Dad has. They only reason I replaced my 4L was because it wasn't economical to repair it and replace the toner cart (which was due). I ended up buying an HP 1012 for $150.