Dude, when you approach the idea of fellowships, you need to ask yourself 2 questions:
1. Will the training change your practice in a way that you will find more enjoyable? All the fellowships can do that for you. However, it's most evident in peds (if you want to be primarily taking care of sick kids), Critical care, and pain as these subspecialties are different enough from general anesthesiology.
2. Will your income increase accordingly to compensate you for that extra year (bear in mind that you are using your year of MAXIMUM income as the opportunity cost - not the 1st year out). This is true for Pain (as of now) for reimbursement reasons and also true for peds, cards, & ICU (because of scarcity of practitioners issues, as those areas aren't known for high reimbursements).
Hopefully, your reasons for completing a fellowship are at least a combination of the 2 reasons above as no amount of money is worth doing a job you are miserable doing (I know a few Pain guys in this category). Your patients will appreciate it as well if you are well adjusted and enjoy what you are doing.
OB - most programs will give you enough exposure to OB to become proficient at bread and butter OB. Fellowships are typically occupied by folks who are interested in high risk OB, or those who are interested in become academic OB anesthesiologists.
Regional - A great choice for those who are interested in getting slick with US guided regional and management of post-op catheters. However, many residencies have faculty that are experienced in bread and butter US guided blocks and management of post op nerve catheters. So a fellowship isn't always worth the opportunity cost if you came from one of these residencies. What IS nice about these fellowships is that many of them will let you be a general anesthesia attending along with your time as a fellow. This lets you keep up your skills and allows you to earn a 6 figure salary as you go through your year of fellowship. Will it increase your earning potential? The short answer is yes. Your group will be able to bill for the US guided blocks. Does that necessarily translate to significant increases to your income? Well, that depends on the paying system of your group. So, it's a tad bit more complicated below the surface. This is my rudimentary understanding of this issue. Any attendings want to chime in?
Not too shabby. Lotsa great opportunities out there. Hope this helps.