Eclipse 4.2 M6 is out and its really feels good to use it. If you thought earlier version of Eclipse 4.2 felt laggy, you should try this one, it has much improved in user interface responsiveness.
For Eclipse 4 RCP there have been also several enhancements and bug fixes compared to Eclipse 4.2 M5 so I suggest you to move to M6.
I also updated my Eclipse 4 RCP tutorial to Eclipse 4.2 M6.
The major concern I heard about getting started with Eclipse 4.2 is that the correct update site for the Eclipse tools fitting to the M-build is too hard to find. Therefore I added the correct update site for the M6 build in the tutorial.
In case you wondering what happened to my planned book about Eclipse 4, it is currently reviewed by a few people and should be available in a few weeks.