There are currently 3 "live" CFWs in use today.
Most people are probably still running some form of M33 (Dark Alex) CFW, the newest of which is 5.00 M33-6. It's starting to show its age, and folks really want Dark Alex to work on a new one, but he seems largely disinclined to do so.
For that reason, plenty of people have jumped into 5.50 Gen-B (an update to the previously released 5.50 Gen-A). It seems to be an overall solid CFW, though it's missing some of Dark-Alex's custom touches (such as a hack that improved memory stick access speeds and an optional plugin that improves legacy homebrew support). Some plugins and apps still work better on M33 CFW (or worse, work not at all on Gen CFW), but it seems that Gen-B is more or less comfortable/safe upgrade from 5.00 M33-6. It bears mentioning that at this point in time, you have to jump through less hoops on Gen CFW than M33 CFW in order to gain access to the PSN and the most recent games (retail and digital). Still, Gen isn't totally required, if you're willing to jump through said hoops.
There's another 5.50 CFW that was put together by some really reliable folks, the dudes behind the Custom Firmware Extender app that helps bring CFW-like functionality to newer PSP 2000s and 3000s. I think they call it 5.50 UTH or something like that. I do not know of anyone (reliable) having tested it out. It seems like the scene is more or less split between those that stick with 5.00 M33-6, and those that upgraded to 5.50 Gen-A/B. 5.50 UTH (or whatever) seems to have made no real splash and been largely dismissed.
For my part, I'm sticking with 5.00 M33-6. My favorite organizational plugin (Game Categories Revised) does not work on 5.50 Gen CFW, and this plugin is essential to me.
Oh, it bears mentioning that true CFW installation (regardless of version) is still limited to PSP 1000s and early 2000s. Later 2000s and all 3000s are unable to permanently install any CFW, at this moment.
Edit - To more specifically answer your question. The official 5.50 and 5.51 updates are not major at all. I think they fixed some new security exploits and added support for subfolders when browsing music/photos. Dark Alex basically laughed at the changes and said from the get-go that he'd not bother to release an M33 CFW based on the 5.50/5.51 updates. He also tried to future-proof his CFW with some new functionality that up until relatively recently, was quite successful at keeping us in the PSN loop. Some of those workarounds have been defeated by Sony, sadly, which is why I say that 5.00 M33-6 is aging. Feature-wise, it's quite excellent, and performance-wise, very stable.
But as always, the biggest reason to ever update your FW is to retain access to the newest and latest games, since Sony keep on arbitrarily forcing folks to update whenever a new wave of games hits store shelves or PSN.
Most people are probably still running some form of M33 (Dark Alex) CFW, the newest of which is 5.00 M33-6. It's starting to show its age, and folks really want Dark Alex to work on a new one, but he seems largely disinclined to do so.
For that reason, plenty of people have jumped into 5.50 Gen-B (an update to the previously released 5.50 Gen-A). It seems to be an overall solid CFW, though it's missing some of Dark-Alex's custom touches (such as a hack that improved memory stick access speeds and an optional plugin that improves legacy homebrew support). Some plugins and apps still work better on M33 CFW (or worse, work not at all on Gen CFW), but it seems that Gen-B is more or less comfortable/safe upgrade from 5.00 M33-6. It bears mentioning that at this point in time, you have to jump through less hoops on Gen CFW than M33 CFW in order to gain access to the PSN and the most recent games (retail and digital). Still, Gen isn't totally required, if you're willing to jump through said hoops.
There's another 5.50 CFW that was put together by some really reliable folks, the dudes behind the Custom Firmware Extender app that helps bring CFW-like functionality to newer PSP 2000s and 3000s. I think they call it 5.50 UTH or something like that. I do not know of anyone (reliable) having tested it out. It seems like the scene is more or less split between those that stick with 5.00 M33-6, and those that upgraded to 5.50 Gen-A/B. 5.50 UTH (or whatever) seems to have made no real splash and been largely dismissed.
For my part, I'm sticking with 5.00 M33-6. My favorite organizational plugin (Game Categories Revised) does not work on 5.50 Gen CFW, and this plugin is essential to me.
Oh, it bears mentioning that true CFW installation (regardless of version) is still limited to PSP 1000s and early 2000s. Later 2000s and all 3000s are unable to permanently install any CFW, at this moment.
Edit - To more specifically answer your question. The official 5.50 and 5.51 updates are not major at all. I think they fixed some new security exploits and added support for subfolders when browsing music/photos. Dark Alex basically laughed at the changes and said from the get-go that he'd not bother to release an M33 CFW based on the 5.50/5.51 updates. He also tried to future-proof his CFW with some new functionality that up until relatively recently, was quite successful at keeping us in the PSN loop. Some of those workarounds have been defeated by Sony, sadly, which is why I say that 5.00 M33-6 is aging. Feature-wise, it's quite excellent, and performance-wise, very stable.
But as always, the biggest reason to ever update your FW is to retain access to the newest and latest games, since Sony keep on arbitrarily forcing folks to update whenever a new wave of games hits store shelves or PSN.
