Panniers won't make you faster.

Whether you're more comfortable is matter of opinion. I prefer not to be the pack mule, and to let my bike carry the load. Others find the subtle handling changes to be more annoying than the load on their back. Plus, backpacks are often a better choice off the bike. Not sure which? There are convertible backpack/panniers.

Is it worthwhile? It depends, I suppose, on which panniers and how heavily they dent your budget. I find the retention systems on inexpensive panniers to lie somewhere between annoying and frustrating. I like Ortleib's retention system best, but they're a whole lotta dough.

I use mine regularly--have for years--and expect to keep using them for many more years. So while my income is very low, their high cost was justifiable, since they turn a routine chore into a pleasure and last practically forever. However, at my income as a part-time civil servant, I wouldn't be able to justify them for only four or five months.

But as Spld says, once you have them, you'll find other ways of using them.