We found ourselves at AK on a very rainy morning on this most recent trip (it rained steadily for about four hours) and while we had ponchos, I found myself really wishing for a rain jacket! The ponchos just didn't keep us dry enough, and they are one size fits all, so I found when the wind blew, it billowed out and the hood blew off and while it wasn't completely useless, it wasn't the best either. The only downside I see to a rain jacket, if you already have one, is what other posters have mentioned -- doesn't keep your lower half dry (especially if you sit on a wet seat!) and is a bit bulkier to carry, and if you are going during the summer, could be uncomfortably warm. Next time we go, if rain is forecast, we'll be bringing lightweight rain jackets AND ponchos!