I've had good luck with the thicker allison covers which are felt-lined on the inside. They are extremely waterproof in my experience.
Here's a few things I've found on both my xb2003 and xr2001 covers:
1. If exposed to a downpour, they need to be on the boat very tight to run the water off. If not tight, the water will pool up and either sneak in the boat somewhere or put a lot of pressure on things like windshields and other rigging. This applies to all boat covers really.
2. If you have a dark hull stripe, the trailer straps are going to scuff them after trailering long distances (2 - 6 hour pulls in my experience). I've tried everything I know of to prevent this, but nothing has worked for me. Now, I am EXTREMELY anal about my boats and a lot of people probably wouldn't notice the marks that i do.
It bothers a couple of my buds, but I keep my boats in the shop AND covered...ha ha ha