how effective are condoms?

The 98% is more to avoid liability from lawsuits, it's honestly more like 99.97. Also, if it fails, it's because the condom broke, so you'll know almost immediately and can go get plan B. Plan B works as long as you take it within something like 2 days, but it's incredibly unpleasant. Use condoms and the pill as a first line of defense. As RA said, make sure it's not expired or torn, and it can never hurt to use birth control as a backup.

"Condoms are 98% effective. The catch lies in the concept of "typical use" as opposed to "perfect use." It makes it sound like "perfect use" is some theoretical expert condom application, where the condom is applied with absolute precision by a team of professional condomifiers in a way that mere "typical" civilians could never replicate.
Nope. All "perfect use" means in the case of condoms is using them every time, and using them for the entire duration of PIV sex. There's a few more catches--you shouldn't reuse condoms, you should throw them away instead of turning them inside out if you put them on wrong the first time, you should use lube but nothing oil-based, and you shouldn't double-bag--but honestly, most of those are statistically minor. The biggest factor in "perfect use" of condoms is actually using them. "
http://pervocracy.blogspot.com/2011/08/condom-failure.html