I had the 2.5" version of the momentus XT on my laptop for a while and it did make a difference to the boot of Windows but after that it was not much; perhaps as some mentioned the 4Gb flash is not big enough and the algorithms not as well optimised but I replaced it with a 240Gb SSD (€189) and the laptop is now a different machine all together! It even allows me to run a Win7 VM at the same time and both OS's run smoothly and fluently.
Anything I buy from now on will come with SSD, I can't see myself going back to spinning disks again not just because of the performance but also for the reliability and battery life!
Market dynamics being what they are, SSD's have come down in price considerably and will probably continue doing so making them more and more accessible and allowing for higher capacities and now that we have mSATA it means that you can have at the same time in a laptop a 128Gb or 256Gb SSD for windows, apps and important files and a spinning disk for secondary stuff. At the moment I use an external disks for that but next laptop will have mSATA and spinning disk most probably.