Last weekend, I visited my FLGS and bought the "Pocket Rulebook" version of Mongoose Traveller. This version is 5.5 x 8.5, the same size as the old classic Traveller little black books.
Having given it a quick read, i'm actually rather pleased with Mongoose Traveller, as it retains the classic Traveller rules and materials almost completely intact. And the price was right, at $20.
My only quibbles -- from my admitedly cursory read -- are that Mongoose Publishing chose to ape the classic Traveller look, rather than putting their own stamp on it ( an understandable decision, considering that they want the product noticed, and purchased, by those waxing nostalgic for classic Traveller ) and they watered down the risk of death in the character generation process. However, they do provide you with an "Iron Man" character generation option in the book, allowing you to risk it all on your survival roll, so the old-school character generation approach is not entirely jettisoned.
Mongoose Traveller greatly expands the available career options to 12, from the original six. That expansion of the career options may have been a feature of MegaTraveller or Traveller: The New Era, but it has been so many years since I last possessed any of those materials that I cannot be certain.
You still have the options of the Navy, Marines, Army, Scouts and Merchants careers. The sixth career, "Other", has been expanded, to include Agent, Citizen, Drifter, Entertainer, Nobility, Rogue, and Scholar. I have no complaints about the expansion of careers, as I don't recall even rolling up a character using the "Other" career path. Providing some definition to that "Other" career path may make that more appealing to players.
Since it's been several years since I last looked at Traveller, I am hard pressed to see any significant differences in the game mechanics between this version and classic Traveller. Thus, considering the modest price, and the handy booklet format, I don't see much downside to using Mongoose Traveller pocket version as your basic Traveller ruleset.
Now, if only I could find time to boilerplate Mongoose Traveller to the 2320 universe near-star map...