I use this station quite a lot. My company is located in central Toronto and I generally choose to get there by train when I visit. The location of the station in downtown TO is well placed as it is walkable for so many things in the downtown area. I can stroll from Union to the hotel I usually use on the west side of the downtown area near Spadina in about 20 minutes. Of course, there are taxis readily available for those who prefer. The structure itself is a classic design and the Great Hall is quite a nice place in which to stand and admire. The station is also clean in general terms and the staff are fairly helpful.
There are a few downsides to this grand place, however. It can get rather crowded at busy times. It is almost certainly used in greater volume than the designers ever imagined and the recent population shift toward downtown probably exacerbates that issue. In that regard, there are also probably not enough amenities in terms of food and beverage offerings. That may simply be due to the fact that they have no place to put them in the more trafficked central areas where they would more likely be patronized. Even train stations like Chicago are better in this respect than TO. The larger central lounge area is not nearly large enough to accommodate the current volume it handles at peak times.
The station also sprawls in every direction because it handles both local GO trains as well as VIA trains. It is not signed well enough as it is easy to find oneself in the local GO train area when one is looking for the VIA trains and vice-versa. The end result is that it is rather difficult to navigate. That situation has improved a little bit with time but not enough.
In general, while there are some very good points about this station, there are also some significant drawbacks. I have learned to use it in the context of my normal routine, which is to ride the VIA in from Windsor and back once every two or three months. I would find it a little overwhelming if I used it outside of that context unless I was using it on an everyday basis and forced myself to acclimate to the station more thoroughly. It's definitely a place where it is wise to allow an extra ten minutes or more if you are embarking on a route for the first time.