It can be excellent exercise if you ride a fair distance (I'd say at least 10 miles) at a good pace (12 miles an hour is not very fast). You're not only working your legs, although they definitely do most of the work. Your heart and lungs are getting a good workout, plus your arms are supporting your upper body and your abdominal muscles help support your torso and keep your back from getting too tired. If you're taking only a leisurely spin around the block, then that's not really exercise in the same sense, but it's getting outside in the fresh air which isn't all bad. If you're not used to riding longish distances, work up to it. Also, wearing bike shorts (with some padding in the seat and longer legs made of close-fitting fabric) will keep your rear end from getting too sore and your inner thighs from getting chafed on the saddle. Have fun!