I used a 500 GB Momentus XT for a bit over a year and recently upgraded to a Samsung 840, also 500 GB. The larger SSDs should be slightly faster than smaller SSDs, and the 500 GB Momentus XT - being the first generation - should be the slowest of the hybrid line.
I made a write-up in the Samsung 840/840 Pro thread, but I'll summarize here: to be perfectly honest, I wasn't blown away by the 840. Moving from the stock 5400 RPM drive that came with my MBP to the Momentus XT felt huge, but the SSD doesn't feel like a huge deal over the Momentus XT. The boot time has been dramatically reduced, but I don't reboot often and that's about the only metric where there's been a really noticeable difference. When it comes to opening programs that I use frequently I can't say that the 840 seems any faster (if anything, based on dock-bounces, it may be slightly slower with certain programs). Of course, the benefit is that the SSD is consistently fast with everything, whereas the Momentus XT had its "learning period" for transferring data to the SSD portion.
Basically, the 840 is faster all-around, but it's not really that much faster for average day-to-day tasks. I was hoping that programs would open near-instantly but it isn't happening. I don't know if it's the drive or if it's a limitation of OS X. There are other benefits, though: my battery life has gained anywhere from 1-2 hours (according to battery estimates; I have yet to run it for that long), I no longer have to worry about moving the computer while the drive is still spinning, and as quiet as the Momentus XT was, there's now zero noise.
I bought my 840 for a good deal ($270) so I don't feel too badly about it. I'll take any speed boost I can get as long as I don't have to break the bank for it. But in the future, I think I'll stick to Seagate's hybrid drives. The cost is significantly less for the storage that you're getting, and the speed difference - at least compared with the Samsung 840 line - really isn't that significant (unfortunately!), unless you find yourself rebooting frequently.