I think it's an upgrade the gives you huge bang for your buck. It makes an immediate and noticable difference. I think it set me back about a buck fifty for some specialized shoes, cleat, and pedals.
I ride on bumpy, rocky, rooty trails with puddles and mud. So, I had a hard time keeping my feet on the pedals. Clipless pedals have made a huge difference. They've also added power to part of the pedal stroke (pulling up).
Unlike many others I haven't had a hard time adapting to them as far as unclipping.
Drawbacks: When I first had them installed I was pulling up too hard on the pedals (intrigued by the new power in that portion of the stroke) which caused some knee strain. So, I've cut back on my pulling up a bit.
You have to walk in those funny cycling shoes when you dismount. My Specialized shoes are pretty comfortable to walk in so, this is no big deal IMO.
It is harder to put a leg out for balance in steep turns. I've had a few slow speed run ins with trees that I attribute to the pedals. I haven't ever had a problem unclipping at stops.
Oh, this is the biggest drawback IMO: If you stall on a steep up-hill, it's hard to get clipped back in to get moving up the hill. You end up walking up the hill a lot of times. You can also stumble around getting clipped in if you've stopped and are starting again on a downhill. For this problem you can buy the kind flat on one side and clip in on the other so you can use it like a platform if you have to. Mine are clip in on both sides which means you don't have to flip them over to get to the clippy part. The guy at the bike shop convinced me that this is better, but I'm not sure about that.
