Well..I've got an early 2008 Mac Pro and a late 2008 MacbookPro.....I've had no dramatic problems with updates to this system including to 10.11.3 ---I have to assume that most folks...considering the numbers of Macs doing the update....pretty much have had a reasonably good experience. There are always a few adjustments, some times lots of them, in operating system upgrades......mostly it will be the folks with problems who bother to check in at forum such as this.
No blamb to the complainers....It's no minor matter when a computer stops working smoothly, or worse, at all.
Computers are as particularly individual as the folks who use them...especially the longer they are used...
No guarantees...but the odds are obviously on your side to have a successful update....also, the odds are that you will have adjustments to make....
I had 3 majorly expensive Windows machines burn out between 1999 and 2008 before I switched to Mac.
Carry on......
illegitimi non carborundum
cheers