I looked seriously into the whole federal law enforcement bit a couple of years ago when I was on the fence about staying in at the end of my obligation. I became an IP at Whiting partially to build hours for the purpose of getting out. I even roped a couple of SNAs to x-country to a Customs aviation unit to tour their hangar, courtesy of a former Marine flying there.
There are a lot of cool opportunities out there. I have nothing but the utmost respect for our law enforcement brethren, who I consider fellow warriors, so to speak (and who would, in a fair world, get all the bennies we get as far as free tix to Busch Gardens, discount movie tix, etc.). Some things that weighed in my decision to stay Marine (errr....). Some may apply to you, some not.
Pay cut--the FBI and Customs air will start you at GS-10, roughly equivalent to a captain. Everyone else will start you as a GS-7 or 9. When you're on the major's list, taking an offer from those would, in effect, mean up to a 4 grade reduction. Most aviators are on the list or close to it by the time they can get out. When I was single and didn't mind living on pizza and Ramen noodles, that wasn't a big deal. Now......
Benefits. Fed employees have good health coverage, but they are on HMOs. See the line above about families. If that doesn't apply to you, good on ya...
Retirement. As stated earlier, you still have to do another 20 years. Every day, that job running an FBO in AZ when I'm only 42 looks a little better.
Speaking of 20 years, one thing that weighed on my mind in regards to the Customs job was the fact that I'd be in the same billet for the rest of my life. The Big Green Gun Club sucks a lot of the time. However, in 1-3 years, you know you'll be doing something else that sucks differently! In a lot of jobs on the civilian side, aviation in particular, once you get qual'ed, you're a line pilot. In 20 years, guess what? You're a pilot with 6000 more hours. In the USMC, even if you don't become a CO or something, you'll have some sort of increasing career progression.
This one is for me in particular, but applies to others in generalized form. I had a MV-22 slot waiting for me around my decision time. No one else has that technology. To broaden that premise, the military has some cool toys, and cool missions, that no one else has.
Only Customs takes aviators and puts them directly into flying billets. The FBI and DEA make you wait 3 years before allowing one to apply for their air units. Border patrol has a different time requirement, but they make you drive the border with everyone else before you can fly, and those guys make less than the GS-7 I mentioned above, unfortunately.
Those are some of my random thoughts on the matter. There are a lot of plusses to going into federal law enforcement. Probably a good 1/4 to 1/3 of my contempories looked into it at one time. It ain't all wine and roses on the other side though.