We have a 4 lamp fluorescent lighting fixture in our kitchen. It had four T12 lamps in it originally. Two days ago two of the lamps stopped working and it smelled bad. One of the ballasts had died.
I took the opportunity to replace the ballast with a T8 ballast and start running T8 lamps on that side. The old ballasts were electronic so they started pretty fast but the T8 starts even faster (fractions of a second difference though). The T8 seems to be at full brightness immediately, the T12 takes a few seconds. Also the T8 is a tiny bit brighter and apparently the bulbs last longer and burn bright for longer, and use less energy (32 versus 40 watts for a 4 foot fixture).
If you have a ballast on a T12 fixture burn out and you don't have a lot of spare T12 lamps to use up (or like us you have other fixtures to use them in) I would definitely recommend moving to T8 and switching all your fixtures over as you replace ballasts anyway. I don't plan to buy any more T12 lamps - when we run out of spares I'll replace the ballasts on existing fixtures.
FWIW the wiring is a little different on T8 lamps but it's very, very simple - anyone who should be touching wiring at all will have no problem with it. There's a diagram on the ballast. Basically you just need one common for all tubes, one separate wire for each tube. T12 usually has two wires going to each end of each tube. So you go from 8 wires for two T12 bulbs to 3 wires for two T8 bulbs. Much less wiring to do. I assume the T8 ballast uses a very high kick voltage at the beginning so the filaments at the ends are not needed at all.
I used a Philips ballast from Lowes - it was about $24. I could have gotten it cheaper at Amazon but I wanted the light back in service ASAP. Payback is rather long on this (at least 4 years) to just replace proactively but if you are replacing a ballast anyway, definitely go this way.