After playing with the settings and testing it out for a few days, I decided to put the 5dm3 through its paces yesterday. Brought it out with the 135L attached, along with the t2i with the 17-55.
After having taken pictures with the t2i for so long, when I first started using the 5dm3, I was blown away. (note that these comments aren't technical at all, more like fanboy-ish haha)
1) The viewfinder is very bright, and the coverage is great.
2) The autofocus is super quick, and can easily be seen in the bright viewfinder. On the other hand, in some instances on the t2i, I wasn't sure if I achieved focus or not because the viewfinder was kinda dark.
3) It will take some time getting used to the different buttons to change the settings, but it's good. One thing I noticed is that with AF on the AF-ON button on the back, I kept hitting the * button instead. I guess my thumb sometimes wasn't long enough to reach the AF-ON button haha. But I have small fingers so most people won't have this problem.
Looking forward to trying out new lenses on the 5dm3!

