Absolutely. First looks do matter and people will be less inclined to check your site out again if their first experience was awful.

An even bigger issue is that if you're entering a crowded market, you'll be signaling to your competition what your approach and features will be. If you've developed something that's not magnitude better, you will be copied and if they have resources, they will outmaneuver you.

I'd suggest people read some of the Michael Porter books on this.