For example, if you know you want to do ECE, don't do the ECE option from Eng Sci... Apparently, ECEs get more technical electives than Eng Scis doing the ECE option
I would like to expand on this point a little bit further. If you know that you really want to do one stream in particular, be it Civil, ECE, Mechanical, Chemical etc. then definitely go straight into those streams. The problem with Engsci is that it covers a broad range of topics, and because of this you don't get the depth you would in the stream devoted to that topic.
For example, if you go into the ECE option through Engsci, you will spend your entire third year just catching up to the students who are in the ECE stream. It won't be until fourth year that you get to specialize in a particular field of ECE. In fact, a couple friends of mine on facebook who are in the third year ECE option were just cursing at the fact that they still had to learn electrical engineering stuff in third year when all they want to do is computer engineering.
There are upsides though too. In second year engsci you get to build a really cool robot, much cooler than anything the ECE stream gets to make in second year (I think).
The nice thing about getting into Engsci is that if you don't like it you can very easily switch into any of the other streams after first or second semester.
The reason I went into engsci (I'm in third year physics option right now) is because it seemed like the hardest math/physics/engineering program in Canada at the time and I wanted to be challenged as much as possible. I also couldn't decide between pure math/physics and engineering so Engineering Science sounded like a good compromise. Oh yea and my cousin was a graduate from the program so that's how I found out about it. For me engsci was an excellent choice and I do not regret it at all.
That said, engsci isn't for everyone and you'll probably find out pretty quickly if you can handle it or not.