Park Tools ones are very ordinary- I saw a suspiciously identical set of no-names at a motorbike show once for a fiver.
These are nice- best standard allen keys I’ve used
http://www.ebay.co.uk/itm/Bondhus-BriteGuard-Ballpoint-Long-Metric-Hex-Allen-Key-Set-9-Ball-End-BLX9MB-/400338292931?pt=UK_Hand_Tools_Equipment&hash=item5d360590c3
(watch out btw, toys’ link is for imperial)
Teng are also lovely, but too expensive IMO- got a load of Teng sockets which are great, the slightly textured finish makes them easier to handle when elbow-deep in oil.
Got a fairly ancient set of Draper Expert handled ones which are just a bit more convenient to handle when you’re not working in small spaces. But the newer ones don’t seem as nice.
Superstar do a set but every time I’ve seen one that’s had a decent amount of use, they’ve been pretty damaged- ball ends missing, hex ends twisted or out of shape.
And yep- if you’ve got a socket set, then a set for those is very useful- good idea to get a long extension bar too. When I’m working on the car or motorbike these are more or less all I use (got a nice t-handle for the sockets too)
Oh and remember, ball-ends can be useful but they’re a recipe for rounded off bolts if used inappropriately.