Positive: very good selection process for photographers with high technical requirements for the content. iStockphoto does NOT charge contributing photographers -- they do if you want to download and use photographs which is of course normal. So it is free for contributing photographers.
To become a member: You'll have to complete online training, pass online "exam", upload three samples for simulated approval, and upload a copy of your photo ID to confirm identity. Once approved, you can start uploading with a limit of 15 photographs per week, which will grow with your status (based on number of downloads). Approval procedure usually takes about one week. At first it seems to go slow but then it gains momentum.
Finance: At first you get 20%. After 500 downloads you can apply for the "exclusive" status (working only for iStockphoto) and slowly increase your slice to 40%. Once you accumulate $100 or more you can request a check.
Now, the most important part: although some users have 1000s of files, probably less than 10% generates them almost all downloads and revenue. For example, my library is only 65 at the moment with 5 photographs generating over 50% of downloads and cash. This one is number one, although not the best in my collection:
Royalty free stock image | Cup of coffee and a business presentation | iStockphoto.com
Also, larger photographs cost more and you could make more money with less downloads if users require better quality for larger formats. This one has generated me most money with half the downloads compared to the previous one, but most downloads were larger sizes (most likely for print):
Royalty free stock image | Wine bottle and glasses | iStockphoto.com
So my advice would be: do not waste your time shooting and uploading 15/week just to keep extending your library hoping that sheer volume would make you more money. Interestingly it won't! Better do just a few at a time but something that will sell well. There are examples of simple photographs downloaded 1000s of times. Check this one for example: over 2000 downloads in less than a year -- the guy probably made around $1,000 on ONLY ONE submission:
Royalty free stock image | wishes | iStockphoto.com
And also follow this guideline: do not design for designers!!! Provide simple and effective content that designers could use the way they want (or just part of it) to produce desired final result. Stock photography is a raw material and support for graphics design, not final design itself. Check "Designer Spotlight" to see how is the content used by designers (very interesting examples):
Design Community: Designer Spotlight | iStockphoto.com
Hope this helps.