Can NFS Shift fill the void left by GT5 delay? No.
But by the same token, I for one am now officially "burnt out" with playing GT5P--again. When I first bought Prologue in April of '08, I played it for about a month (offline) and got completely bored to death with it.
Then, I discovered GTPlanet and started playing online for the first time in late March of this year. That was pretty enjoyable for about three or four more months, then the boredom set in again about 2 months ago. If not for online events such as Suzuka 750 PP and 800 PP as well as the Weekly Race Series, I would have again been bored to tears much sooner. Having had my second go-around with Prologue now, I am once again completely and utterly bored by it.
So in essence, Prologue is good for about a month of offline play and about 3 months of online play--for me. I certainly can't speak for anyone else.
Now, let's look at Shift. The majority of people responding to this thread are bludgeoning Shift as though it is a pile of rubbish. Pretty much every review of the game has been positive. Just look at the following list:
Worthplaying.com--8.5/10
http://worthplaying.com/article/2009/9/27/reviews/68823/
IGN--9/10
http://pc.ign.com/articles/102/1023587p4.html
Games radar--9/10
http://www.gamesradar.com/ps3/need-for-speed-shift/review/need-for-speed-shift/a-2009091516453418594361/g-200901301742944010
G4TV--5/5
http://g4tv.com/games/ps3/61972/Need-for-Speed-Shift/review
And if that's not enough, just take a look at the poll results in
this thread
here at GTPlanet. Approximately 80% of the people who voted in that poll gave Shift a rating between 7 and a 10 out of a possible 10, with about 63% of those voting scoring it between an 8 and 10.
Bottom line: Shift is an excellent game. The offline features should last me about a month (the same as Prologue). It does several things better than Prologue: The sounds are excellent--a noticeable improvement from Prologue, where some of the cars sound like their engine compartment is comprised of a vacuum cleaner or coffee grinder. Cockpit view is better--the best I've seen in a console or PC game to date. Certain elements of the physics are clearly better, such as the sense of weight transfer in the cars, the braking physics and the sense of speed. The online play is better than Prologue. No lag. Customizeable events. Private rooms. The graphics in Prologue are only marginally better than those in Shift.
I just find it shocking that so many people are so dismissive of Shift

If you have actually played the game, and have these opinions, fine. I can't argue with that. But if you are basing your opinions on rumours or youtube videos, then you have no idea what you are talking about.
Also, if you are playing the game with a controller, you are only playing half the game. Sorry, but it's the truth. Play the game with a wheel and see what you think of it then. You can get yourself a good wheel setup for about $60 (DFGT or DFP on eBay + IKEA table or old computer desk at a thrift store, or any other stable mounting solution), and it will increase your enjoyment of driving games by orders of magnitude--one of the best purchases you will ever make. *For comparison, a brand new Dualshock 3 controller costs $50 at Wal-Mart here in the U.S.*