I've contributed a few times to a chipin, but like rather to contribute and communicate directly with the maintainer for certain specific features. 

I think there are 3 main ways:
- chipin: for popular much wanted features, a lot of people contributing a small amount. 
- bounty: gathering a few sponsors that contribute a larger amount for a specific feature
- direct contact with the maintainer: for urgent needs or really custom changes to the module for a specific project.

I think chipin is suitable to gather a rather small amount with a lot of people giving very small amounts. So its mainly suitable for popular much wanted features, not something that only a few are really interested in. For my company I would like to have an invoice of expenses and with chipin this is more difficult. Also if you contribute a larger amount it would be nice to be mentioned as "sponsored by" on the project page; with chipin normally every contributor is anonymous, if you pay half of the total amount or 5$.

I like the idea of "bounty" more for specific stuff, so gathering with a few sponsors and pay each a third or so, but it could be good to find a better way to show clearly which bounties are searching for extra sponsors for a certain module. There should somewhere be an overview. Same would be true for chipins if you would have more.

For me it would be also good if contributors could state in a consequent and clear manner that they are available for paid customisations (and quite a bit do this allready, but maybe drupal.org should provide an structured way to do this) and give in their profile some more info about conditions, contact ways, timing and availability. Also to know if the maintainer is a fulltime freelancer available or somebody working fulltime in a non-drupal related company having only some weekend time to spend and no possibility to write an invoice is very important to me.

Hans