You'll find a fair few comments about Photographers Direct (PD) from photographers who DO use PD and other agencies, spanning the past two years or so, on this UK Forum called StockAgencyForum- http://www.stockagencyforum.com/
I'm not with PD myself (I use a couple of other agencies), and - although I'm a member of that Forum - I'll let the other chaps speak for themselves rather than paraphrase them. I trust you'll accept that
To see the "Agencies" thread and discussions on that Forum you'll need to register, but it's the usual straightforward e-mail-acknowledgement registration scheme.
It's a familiar phpBB-based Forum and its Search tool works well.
Hope that helps,