I have been doing stock for 4 months or so and do submit to istock.  I basically use up the 18 per week allowance by submitting 3 a day, which takes about 5 minutes and means you build up a portfolio slowly (50-60% acceptance rate for me).  Once I passed 100 or so in my portfolio sales become more regular (1-2 per week, so not loads).

As a relative newbie to stock, I would say yes, it is worth joining but don't rely on istock for a lot of income unless you go exclusive (which takes time- you need ?250 downloads).  Saying that I have had more extended license sales there than elsewhere.

I definitely recommend shutterstock-  they have very tough selection criteria and getting accepted is tricky but once you're in you will develop fast- the 3 day review times and unlimited uploads are advantages and you will learn what does well.  Since developing my portfolio there, I have done a lot better on other sites- it is like being on a training course in itself.

Good luck
Another Tom :-)

exactly,  if they had been more like  "we charge this much per year and this is why it is worth it" in the early stage of application,  I might have been more convinced

That sounds promising.  I am considering Veer..

Do you mind sharing roughly how many images you have in your portfolio?