I've used the Danuu cover for years. I don't have any storage space in the house so I leave my kayak on my truck. The cover is great for keeping the kayak shielded from the sun. It is somewhat waterproof, but not totally.
I have found that you will have condensate build up in the hull regardless of whether it is covered or not. The kayak is not air tight, and will always have humid air inside. When you leave it outside it will go through the normal diurnal heating and cooling effects. This will cause condensation to form inside the hull. Keeping it covered will help reduce the amount of condensation, but will not stop it. When it is warm and humid I take my hatches off and leave the cover on. This allows the kayak to air out a little better. Keeping the kayak covered and in the shade is the best thing you can do.
Be careful when strapping the cover on if you plan on driving with it attached. The cover blows around quite a bit at freeway speeds and the constant rubbing can cause problems. The straps for my cover run around the bottom of the large kayak rack that I use. This keeps the straps from rubbing on the kayak hull. I had an extra bungee cord holding down the cover near the stern of my kayak. After a trip up to Monterey I noticed that the bungee was rubbing on the keel. It had worn through the JB Weld skid pad on the keel. I'm glad I used the JB Weld. Without it, the bungee would have rubbed a hole in the keel.
The only problem that I've had so far is that the cover wore holes right through some of my hatch bungees. Driving with the cover on sucks, because the cover will wear out and tear eventually. I think I'm on my third cover so far.
A cover is a good investment for keeping your kayak in good condition, but you'll still have to do some regular cleaning/maintenance on it.