Update 2: Nelson, Michael Bay's site admin, posted "Their is a script out there being reported on TF sites as being legit. It's fake. Amazing that someone would take so much time to write so much for nothing." Thanks to Dudelsack for the link.
Update 1: Here is a more detailed description and analysis now that I have had time to do more than just skim the script.
The script is essentially a love letter to the history of Transformers. There is a jump to the past so some characters both new and old can get a makeover in their more famous alt modes. In some cases there are literal re-creations of moments from the generation one cartoon series (especially "More Than Meets the Eyes") and Transformers: The Movie. Most of the new characters are classic G1 with Percepter, Laserbeak, Mirage, Hound, Thundercracker and many more. The script has some rather clever ideas and plot twists such as given Prime a trailer using Mirage's holographic technology, the introduction of a new plot McGuffin with the Key to Vector Sigma and retroactively correcting many of the plot wholes from the first movie. The final act has a moment to up the Devastator ante with Superion vs. Bruticus while having Prime and Megatron once again go at it, with Soundwave joining the battle with his mini-friends (Ravage, Rumble, and co). The final scenes make for an excellent cliffhanger moment. It was an enjoyable read that gave me some warm and fuzzies as I recognized many of the moments from my childhood love of Transformers.
Having said that, all that is also the very reason this script is NOT the real deal. Michael Bay has made it clear he doesn't really like doing homages, limiting it as much as possible while this script is just thick with it. The ideas, while huge and interesting, have rather long sequences (including a space fight) being pure CGI and for most scenes the Transformers are almost constantly in robot mode. Part of the reason for the emphasis on human interactions, human technology and the vehicle alt modes is it saves on the budget. The final pages, with the cliffhanger, also breaks a Bay rule of keeping movies contained to itself.
Finally, this script doesn't have that "emotional hook" that Bay, the writers, etc loved harping on (in a silly attempt to elevate their work to more than it really is but whatever works I guess). For the first movie it was a boy and his first car, the second was boy becomes man as he accepts his responsibilities, and this one has a brief moment where Sam and Michaela become engaged but nothing else really.
Overall, I was entertained by the script and would not mind this being the real deal. Sadly that is not the case. Maybe IDW can do an "Elseworlds" type thing with in it comic book form. There was clearly a lot of thought put into this so thanks to the writer for putting together an enjoyable read.