So far every game I have actually followed the making of and news, has been no less then good.
Of last year, the two I followed the most in the making, was Pokemon White, and Dragon Age 2.
Both turned out to be fantastic. DA2 was #2 in my top five games of last year.
Though their haven't been that many game purchases that I've been disappointed in.
My first real disappointment for my 360 came when I pre-ordered the collector's edition of Star Wars: The Force Unleashed 2 in 2010. If you don't horse around and try to find collectables and power-ups, it is a four hour game at best. Though points were gained through the later special costume DLC only being a dollar, and the Endor DLC allowing me to Force punt Ewoks into the sky, never to return.
Another sort of recent disappointment purchase came after I followed and decided to pre-order Dragon Age 2, but the disappointment didn't come from that game. I thought since I hadn't played DA:Origins yet, I would purchase the Ultimate Edition of it and try to finish at least one play through before DA2 got released three months later. I just couldn't do it, most everything about DA:Origins is just hosed up and outdated, even for the time it was release. I could only get in a little over 20 hours of gameplay, before I gave up because I wasn't going to wade through it just for the sake of getting through it.
Luckily I decided not to cancel my pre-order of Dragon Age 2, because I knew from following the making, that they were making a lot of fixes of mistakes they made with DA:O, and that it was a different type of story. I was thrilled with that purchase, because DA2 turned out to be worlds better than DA:O; BioWare had fixed all the things I saw as problems in DA:O, the only flaw was the semi-repeat dungeons, thought since the rest was phenomenal I didn't mind.