you can read up on the benefits in game where you sign up, disadvantages are people like me will attack you if we find you. You Fed lover you!
^ and me gonna join as a purple disadvantage [alien]
The only benefits I see are for whatever doo dad they are giving away after 4 weeks. I signed up for Aisling to get the shields, did nothing until the 4th week, then went out to an Anarchy system in an opposite faction and generated 10k merits killing the powerplay ships, would camp out at a big station and wait for them to try to land, then kill them, but you need a fast ship with a lot of Alpha damage, because the moment you open up on them they will try to run and jump out, and NPCs aren't hampered with silly things like "Mass lock" like the rest of us, had one NPC jump out from right next to the station, I was shooting at him, and I was mass locked, even if I had a wake scanner, I would not of been able to jump from his wake. Made 50 mil when the tick occurred and used that bonus to buy and stock some Prismatics, then didn't do anything else and the next two ticks made 5 mil each, then when eventually my merits ticked down to zero, left the powerplay. I didn't defect, because they send bounty hunters if you defect, but if you just leave the powerplay group, they leave you alone, wait a day or two and join the next powerplay and start the whole process over. I joined the one that gives away the mining lasers, figure they would be better than the standard mining lasers, maybe a little extra range to go along with the damage they do against all targets, would be nice, but don't think they have an increase in range. But figure will get them to put them on my Cutter mega mining ship, and the Fed Drop Ship mining, use the FDS for short mining missions and the Cutter to go rake in major loads. So in 2 weeks will find an Anarchy system nearby and take the Corvette out and do the 10k merits thing again.
Err.... you only need to be aligned for 3 weeks, then gain 750 Merits during the 4th week so at the end of it you get Rank 3 & quailify for the toys.
Err....sorry, misspoke, that's what I meant even though I didn't say it right, it may be time consuming to shoot for the 10 k merits in that week cause most the time you're looking for the powerplay ships in supercruise and interdicting them, which isn't difficult, just time consuming. So I routinely SC around a large station, get a couple, then reenter station area and hunt for the ones that show up there, then back out to reset the instance. Seemed to be quicker that way, didn't have to wait as long.