My experience so far, working with ~4 add-on developers on XF -
1.
Developer 1 : Doesn't ask for money until project is done. Works on it for 3 months then vanishes off from the Internet
2.
Developer 2: Starts working on the project by committing that he'll ask for full money when the project is over. Makes lot of progress and then becomes totally unresponsive for 2 months. I make phone calls, skype calls but no answer! Then one day pops-up and says he wants to finish off the work he started. We start the project again, he works for 1 week and then vanishes again for 1 month.
Frustrated, I find
another developer. Now this 'Developer 2' pops in again and says "You *MUST* pay me for the work I've done".
3. Now this 'another developer' from #2 above, says he'll be free after several, unknown number of weeks. So I find 'yet another' developer 'locally' and then he writes to me the very next day saying he's free. I've already awarded the work to Developer 4.
4.
Developer 4 works on the project without asking for money for full 4 months ---- makes 90% of the add-on; then suddenly stops answering phone calls / emails / IMs / conversations.
...and I'm still looking for a developer who'll do some magic.
(PS: Just received a conversation from yet, yet another developer who says he'll be free after 2 months

)