I've never used TopTal (or any other agency for that matter, since I'm a freelancer myself that finds my own clients), so obviously don't have any insider information, but here's my impression. If you listen to Mixergy, TopTal is a big sponsor, and also their founder has done an interview on there. They claim that they have a really great interview process (their secret sauce) to find talented developers. From what I've read in blog posts from people who have gone through it, it sounds rigorous, but in a very annoying, time-wasting way. However, that's typical of a lot of big tech companies these days (so if they're similar to, say, Google, in recruiting, is that a bad thing? Maybe not from your perspective). My personal opinion is that those methods will weed out both good and bad developers. I would probably fail those interviews, but my clients can't get enough of my time at an above market rate - I'm a good communicator and deliver quality, on time, but I haven't studied a CS textbook in over a decade and don't do well at riddles. I also could not be bothered to do a multi-day, multi-phase, multi-project/homework recruitment process unless I'm being paid significantly for it. I'm no unicorn - there are lots of developers like that out there.
One thing that I think TopTal has going for it, is you pay a lot more than those other sites. The few freelancers that I've read that work through them claim they are making more than their market rate when on their own (which may say more about those freelancers than the market). However, generally, the saying that you "get what you pay for" is true. There will be exceptions, but if you're paying $100-150+/hr for a developer, they will either deliver or you will cut them loose quickly, because of the high bar set by the rate. With a $40/hr developer, it's much easier to let them drag on because of the low expectations set by the rate. So I'm not really recommending TopTal, but I do think that in general finding a freelancer with a higher rate will tend to be some insurance that they'll deliver. I am, of course, highly biased in this respect, but I'm not soliciting my services (I'm actually trying to reduce my freelancing workload to dedicate more time to bootstapping!).