Sienna seats are very high quality and very comfortable. They are one of the nicest automotive seats I've ever been in. One of the only seats out there with built in foot rest too.
I have 6 of them in my van. They are an incredible upgrade over the stock seats.
2011-2014 all have the same upholstery style and colors (light grey, two-tone grey, light tan, two-tone tan). 2015 has new colors and new styling but the base of the seat is essentially the same as previous years. I'm planning on upgrading my seats to the 2015 grey/black colors at some point.