The one who will be dissapointed, are those who are listening to bias reviewers, nitpicky freaks, and fanboys.
Untrue - you can be disappointed without being biased by the opinions of others or be a "fanboy". I have only owned a PS3 since Dec 2009 and an Xbox since about June 2010, so I have no strong "fanboy" attraction to one or the other, nor have I been waiting 5 years... or played GT1-4 at all.
GT5 is a flawed gem - at times it shines, but at times it really disappoints. Perfect example - Top Gear Test Track. Everyone wants to dream of being the Stig or beating the times from the show in the reasonably priced car, yet we can't because nobody at PD thought it might be a good idea to give us the reasonably priced car or allow standing starts for lap times!
How hard would it have been to integrate a feed of lap times set by the guests on Top Gear (updated every so often too) so that you could watch the show and then go run to your game and try to outdo the latest and greatest time in both the reasonably priced car or the latest exotic. THAT is disappointing.
There are many very valid complaints on this thread - not just "why no porsche doods!!" comments - but careful observations from obvious fans who can't understand why some simple and obvious things weren't caught in quality control or improved based on innovations of GT5 competitors.
Really - which would you rather have for leveling up or winning: a new horn or a percentage discount on tuning part purchases? Clicking through 6-7 menu screens to get the right car for a race or the game giving you a list of only the cars which qualify? 800 (sub)standard car models + 200 ultra detailed models or 700 total all done to the same standards with interior views and the same tuning/wheel change options?
For everything I love in GT5 (and there is plenty) there is always something that really diminishes the overall experience - and it is usually minor things which become big things simply because you start thinking "Why wasn't this caught, with all the time they have been working on this and all the promises of delivering so much".
Where is the logic in unlocking damage only at level 20 or 40? I told my wife about that "feature" - she has never played a video game nor cares to - but even she got a strange look and said "why would anyone want to unlock damage as a prize -shouldn't that be part of the simulation from the start?" maybe this unlock system works when you are 19 and can play until 3 am - but when you are 39 and have a wife, kid, job, bills, etc like me you get to play MAYBE 2-3 hours a week tops. I'll see some of these so called features around 2012.
I'll keep playing GT5 and enjoying it for the most part - but it sure doesn't make me want to give up Forza 3, Dirt 2, or even Need For Speed Hot Pursuit... and I'm looking forward to Dirt 3 and Shift 2. GT5 is just another option - not the best standing far above the rest as it aspires to be - THAT is the disappointment.