Two different brushes on test here; the Bike-it / Tirox 360 degree chain brush (around £12 from Ebay) and a generic chain cleaning brush (from £3 upwards on Ebay). For testing purposes, I used WD40 as the cleaning solvent for both brushes and I know there are often arguments about WD damaging a chain's rubber O rings, but I've been using it on my bikes' chains for the last 30 years without issue.
First up, the blue U shaped chain brush. There are lots of versions of this brush about, brand name versions that tend to be more expensive, and many (like this particular example obtained via Ebay) cheap, unbranded versions. The bristles on this are quite stiff, but not so stiff that I’d be worried about damaging the chain O rings. It’s operation is fairly self explanatory. Shove chain into U shaped brush and scrub away. My chain wasn’t horrendously filthy, so there’s not a dramatic difference between clean and dirty. But, the chain is cleaner and the brush made a reasonable job of cleaning between the side plates. If you wanted to do a better job of them, you could always use the other end of the brush and clean each individual gap but that is time consuming, especially on the other (wheel) side of the chain where access is difficult. The obvious disadvantage of this brush is that you can only do three sides of the chain at once. The fourth side can either be done by putting the U end of the brush back on the chain the other way up, or use the other end of the brush to finish off just the fourth side. For a couple of quid, though, it does a pretty good job. Not sure that the more expensive versions of this brush would offer you any better result (for example this is Laser Tools' version from Machine Mart) but as I haven’t actually tried them, who knows…

 The Bike-it / Tirox 360 degree brush is designed to solve the main drawback of the U brush and can clean all four sides of the chain at once. It is, however, more tricky to get it on the chain.
After working out that you need to extend the brush and then spiral it onto the chain, it’s then a matter of applying your chosen cleaning product and scrubbing away. But, it’s harder to use this than the U brush. With the U brush and the bike on a centre or paddock stand, you can hold the brush in place with one hand and turn the wheel with the other. I can’t do that with this brush because its grip on the chain is so firm. The brush really needs two hands to move it along the chain, but with the bike on its centre stand, trying to move the brush just causes the wheel to turn. I ended up in a pose reminiscent of a game of Twister, with a foot trying the wedge the rear wheel still and two hands trying to move the brush along the chain.
After cleaning the exposed length of chain, the wheel then needs to be turned and the process repeated. A little more effort and faff then, than that required for the U brush. But it did do a better job, cleaning in between the side plates as well as the more easily accessible outer chain plates.
The crunch question then, which is best?Well, the 360 brush did the best cleaning job, but, personally, I’m more likely to use the tool that is quickest and easiest to use, and that’s the U brush. I could give the chain a reasonable clean in half the time compared with what it would take with the 360 brush and that means that I’m more likely to actually do it!

If you want to go really flash, Machine Mart sell a fully enclosed chain cleaning bath, but at over £40 I'm unlikely to be trying that one!