I recently bought one from here:
http://www.ebay.co.uk/itm/neilsen-42-towed-lawn-sweeper-next-day-delivery-/301611978037?hash=item4639798d35
We have a meadow at the back of our house which is cut by ride-on lawn-mower. When we changed the ride-on OH decided that we didn't need the one with the collection bag! Then he decided he didn't like the look of the field when it had strips of cut grass lying on it, so took to raking it up! Hard work and what's the point of straining yourself for the sake of £200?
It came very quickly by courier, he carried it down the drive on his shoulder and it weighs a ton! It needs putting together which OH did a lot of swearing about - men and instructions! It is time consuming to put together and the brushes come covered with bolted on heavy cardboard which caused some confusion in OH, as the instructions don't mention that, but logic tells you to take them off - doesn't it?
Once put together it is sturdy and works very well and as our field hadn't been raked properly for ages, caused more swearing about having to empty it! The emptying just involves pulling the string which lifts the back of the bag and dumps the collection. We bought the 42" size but they come in other sizes, anything bigger seemed pointless as the cut of the mower meant the strips wouldn't be covered two wide, but I suppose it would brush the cut bit.
You can also buy an 'off-set' type which is for a side ejecting mower (which ours is) which you can then tow behind while you are cutting and collect at the same time. These are more expensive and as you are just riding about on the mower the one we bought does the job perfectly. If you have a back ejecting mower you can use this one at the same time.
So our grass gets cut, left to dry off a bit and then OH rides about on the mower and collects the grass, no more strain on his back and he's happy with the look of the grass!

Thanks, that's very useful and I've ordered one.

Super, best of luck with it. We are happy with ours. Happy to help as I took lots of time dithering about whether to buy one, also not knowing anyone who had one to ask!