It's the same flea ingredient as in the prescription-only Advantage Multi (which also covers heartworms, unlike Advantage II). It's VERY effective in my region. Like Frontline, it's been around a long time so some fleas in some areas may be resistant to it -- luckily it still works for me, but YMMV. (OTOH, Frontline seems totally worthless in my area.)
If your vet is selling it, odds are it's a good product for YOUR area, as the vets tend to know exactly what works (and what doesn't) locally. FWIW, if it works well in your region, and if you also are buying separate heartworm prevention (like Heartguard), you can likely save money by switching to Advantage Multi which takes the place of both because it includes a heartworm prevention ingredient with the flea prevention (I pay about $13 per dose buying from Valleyvet.com or KVsupply.com).