I've been using Commision Junction for some time so will take a shot at answering your questions.
1. What is properly qualified as a pay-per-lead?
The definition of pay-per-lead varies from program to program. My definition is that the user clicks the link, enters the target site and does something. For example signing up for a newsletter, downloading a file, requesting a brochure etc.
2.How long do they take to update the revenue earned...
I've found it usually updates the next day.
3.Are they reliable?
I've had no problems so far. Click throughs are tracked properly etc. I think sometimes their impression tracking is a bit off though, so I use AdCycle to run the cj.com ads and track click-thrus / impressions myself.
4.Which kind of ads on their site are 'hotter'?
That depends on your site. I've had most success with 468x60 banner ads and text links. Text links incorporated into the body text of an article of paragraph seem to do well.
5.How long to they take to send a check?
Got me there, I've not earned $25 yet :). Anyone else know?
Why does my English go to the dogs when I try and touch type?
<Edited by Fluffykins on 01-19-2001 at 04:02 AM>