Good design and flow is critical, especially for consumers apps. I'd say that 2 of 3 of our founders consider design as one of our core competencies and I think its made a huge difference.
But even if its not a core competency, make it one of yours! Our 1 non-designer cofounder learned about and talked to other startups about design, and ended up coming up with our biggest design improvement to date - removing the whole "website" from the frontpage. If you go to Anywhere.FM, you'll notice we don't take you to a "website" and this change reduced our bouncerate by 50% (compared to the Alpha release). This certainly trumps most technical feats we've accomplished to date.
Finally, its good to have multiple opinions (but not get locked up in minor details), so that the UI targets a wide audience - not just the audience the designers are most in tune with. Its easy to target only yourself, and we probably still suffer from that today.