I'm going to preface my review by noting that I am not a daily commuter during rush hour, which I think would add some frustration...though I think that all commuting can be frustrating.
What I love about GO transit is the cheap access to a growing number of places within and on the outskirts of the GTA --Hamilton, Niagara Falls, Peterborough, Cambridge, Waterloo...I could go on. It makes it really easy to visit friends and family without trying to beg a ride or rent a car. There are bike racks on ALL buses and you can take your bike on the train so long as you're not arriving at Union between 6:30 and 9:30 am or leaving Union between 3:30 and 6:30pm on weekdays. If you like to cycle and don't want to spend half your day getting out of Toronto, GO transit is a great way to access some routes or use it for one-way of your trip.
I'm surprised by the amount of negative comments about the customer service. Every single time I use the service I am taken aback by the level of friendly helpfulness sent my way. The drivers and train crews are really cheerful and often sing/make jokes while calling out stops. At Union station, if I'm squinting up at the schedules I am always asked if I need help by a GO transit worker who genuinely seems to enjoy providing service. The one exception here would be the ticket-tellers, my experience there has been hit and miss (sometimes they're pleasant and other times I'm greeted with a scowl and offered misinformation) --just step away from the booth and you're likely to find someone helpful.
I travel by GO on average once a week and have only experienced two short delays during the five years or so that I've frequented the service. I'm sure I'd be more annoyed if it made be late for work but I would MUCH rather get stuck on a train for a few extra minutes than have to sit in traffic for any length of time. Prices may be more than you would pay in say Europe for comparable service but in Canada it is a total bargain and way, way cheaper than the VIA.