I feel the opposite. I bought a Roomba 650 almost exactly four years ago (one of the best purchases I've ever made) for about 350 dollars after a bed bath and beyond coupon. With a little bit of maintenance it still runs every day exactly as well as it did the day I bought it.

The 650 is the robovacuum Sweethome recommends and has recommended for years (http://thesweethome.com/reviews/best-robot-vacuum). The newest Roombas since then have been mildly incremental, and the competitors have struggled to offer something that works better. One of my close friends bought the exact same model last week for 370 dollars.

That's not to say there isn't room for a lot of improvement, but I think it's been a super slow moving space.