The Bike Cave. It's a good name for this product because it's a great, big cavernous thing. You could have a branch of Al Quaeda hiding in there and you wouldn't know. If other user reports are to be believed, it's also slightly damp, although, in fairness, the manufacturer never claims complete water-repellence.
However, unless you have a garage or a massive hallway, it's still the best option there is.
I had previously spent £20 on a single bike cover and it had worked relatively well. However, it didn't have the coverage of a tent and, over the course of a year, I noticed the bike was starting to rust where the lower chain and spokes were exposed to the elements.
When we purchased a second bike, we therefore decided to plump for the Bike Cave.
The Bike Cave is actually very good value. It's the same price as two waterproof covers and it has a base that means you can also store bike accessories on the floor. It's also easy to assemble and move around.
The tent comes with two ballast bags so you can weight it down but, unless your bikes are made of balloon, it's probably not going anywhere. I've also got enough bits and pieces stored in the base that it still sits stock-still when we take the bikes out.
There have been complaints about the zip but it's only problematic around the lower line due to the doubling-up of the hem. This isn't insuperable and you simply need to take it easy as you close the flap to avoid snagging the extra material.
Is the tent perfect? Not really. It would've been nice to have more access points so it was possible to take an inside bike out without taking them all out. However, is it better than anything else that's out there? Well, probably, yes.