I think Awin are my favourite network at present, but that is from a designer point of view due to they're ease at using datafeeds by means of the tools that they provide.
With direct PPC they have a deep link generator to use for each merchant, but it all depends on acceptance to be able to use it. I have been accepted into way more programs than I have been declined and they pay on an optional 2 week or monthly basis, prior to a met threshold minimum of just £25. I suppose a nice email to the merchant will help you on your way to acceptance if you give them a layout of your plans.
Aff Future also have a good acceptance rate and make it easy to deep link. Payments made at the end (ish) of every month.
I used to like tradedoubler, but as I use many sites I used to have to get each site accepted into just 1 program (something to do with click tracking), but that was a while ago and things may have changed since. I also didn't like that I was waiting for my commissions 2 months after, but that is just the way they work and lots of affiliates seem happy with that, i.e:- they are not working to a minimum budget so can afford to wait. They have many plusses though too, so I dont wish to drag they're name through the mud

Its just that I have found my home with Awin and they're services and I feel they cater for all areas of web promotion, like PPC,
SEO, Datafeeds, etc giving them a complete module that is fit for all types of affiliates. I also like they're detailed click, commission tracking reports and they're control panel is user friendly beyond belief.
I also have to comment on they're support. Over the last month I have sent about 10 tickets asking questions regarding merchants, and they always replied within a day.
I believe buy.at to have a good reputation, from what I have read anyway. But I have never used them so cant really give them a positive comment.
Hope that helps,
Terry