Does anyone use them? Are they easy to use?
Yes. And yes. There are a few types of 'easy-put-up' designs, and I wouldn't buy anything else. There's the umbrella type; the fold-up springy ones; and others that use a conventional speedring but with permanently attached rods that are hinged and pushed/clicked into position individually, but it's very easy. This latter type is arguably the better as it avoids having the umbrella mech in front of the flash tube. This shades the light from going directly forwards, but in practise that makes no difference at all with a normal studio-type head. Not ideal if using a speedlite though.
The type to avoid are the ones where the rods all have to be threaded up around the front and then secured to the speedring/base. There's a knack to it, but it's always a PITA and it's usually difficult not to put stress on the ends of the rods that can rip the material. On the other hand, if you are able to leave the softboxes up and assembled, then they make very good lights.
I usually stick to umbrellas as I'm a mobile photographer, I just find them easier but have been looking at the easy open ones.
I've heard that you can just about get away with unfastening a small amount of the diffusers to get your hand in and collapse them, is this right?
Umbrellas are great. There's nothing easier (or cheaper) and the light is, in a lot of cases, indistinguishable from a similar sized softbox
The main 'problem' is spill, which is something to be aware of, but often not a problem, sometimes beneficial. Reverse-firing modifiers, eg umbrellas and a few others, are also more space efficient and can be used directly against a wall or ceiling, which comes in handy in a lot of normal domestic rooms. From a lot of the advice round here, you'd think that the only light worth using was a softbox with a grid, but that's far from true: only use grids when you need them, not otherwise - they eat light, and make the light harder as the grid blocks the light from the edges of the softbox.