For Christmas I got a new pair of Altra 3.5's. Now the 3.0's have been better then the 2.5's and have been the best shoe I have ever used on the trail. So I wear a 9.5 for an everyday Altra (2.5's) and with my hiking Altra's I have always worn a 10 and they work great, plenty of room. With going into my third set of trail Altras I naturally got a size 10. They were smaller then my everyday 9.5 2.5's felt! My toes were completely flush at the front of the toe box. Returned them to amazon on Dec 25th and 2 days later my Altra 3.5 size 11 were on my front porch. Fit just like my 3.0's in size 10.
Now other then the size downscaling that seems to have happened, the 3.5 has a rock plate in it which I am pretty happy about seeing as how I have PA coming up in July. But as well the other noticeable difference is the large amount of padding in in the shoe as well as what seems to be a better grip on the tread and the durability of the outside of the shoe.
My advice for those looking to transfer from the Altra 2.5-3.0 to the 3.5 is to go up 1 full size from what worked best for you in the previous models. Honestly if the 3.0's had a rock plate in them I would be ordering another pair of them. I will update this thread once I have miles under the shoe.