Personally, I’ve never used a rain cover for any of my backpacks — they’ve just always seemed unnecessary. Most contemporary backpacks are pretty water resistant. If you’re out in an all out downpour for a long time, sure, they’ll leak, but if you’re just going from place to place (like from a hostel to a bus station) they usually stay pretty dry inside even in heavy rain. That being said, I don’t think rain covers are a bad idea if you’re out hiking or trekking regularly. For the amount of money that you pay for them — under $10 — it may be worth it for the peace of mind.
One thing that I will do to keep rain off my gear is wear a poncho over myself and backpack. This works. But, again, for the $10 a rain cover for a backpack costs it is probably not really worth the hassle coming up with alternative strategies. Also, one of these rain covers are usually good to use for many years. You’ll probably end up losing it before it breaks down and starts leaking.
Though I do caution against putting your backpack in a rain cover then storing it in the luggage hold of a bus. Take the cover off first, as it’s a ridiculous situation to have to go crawling around under a bus because the rain cover popped off your backpack in transit.
Then again, I usually keep my electronic gear and things that would get damaged in the rain in a waterproof messenger bag, so whether or not the gear in my backpack stays dry or not is a matter of annoyance, not an all out tragedy.
Here is a backpack rain cover for $8 for sale through Amazon:
Frogg Toggs Backpack Cover-LG
What’s your take, do you use a rain cover for your backpack?