I went there today because the site is small enough that I could get to it and see the whole thing between the time my classes ended (5:15) and the time it gets shut for the night (8:00). The Temple was actually quite small, I thought. All the doorways where very narrow and on the second floor my head was barely clear of the rafters. The stairs were tight enough that they had to put guards at both ends to direct traffic up and down. There was not room for two people to pass each other on those stairs, even if they were really good friends.
On the other hand, from the outside it looks pretty cool. It's in a park on a hill, so there are nice views of the city to be had from it, especially around sunset, when the locals come out to enjoy it. Everything is pretty well explained, if you can read Spanish. The English versions were less helpful. The also had a pretty nifty system of lights projected inside, which highlighted the architecture, and explained what it was. I dumped a bunch of photos into the bucket, and I think this is once case where the pictures definitely do a better job of explaining what I'm on about than I can do on my own.
Most importantly, at least to the view of this poor student, entry to the temple is free. I took the metro to get there, because I was coming straight from class and had a backpack full of books. Someone with a more touristy agenda could walk from Puerto del Sol or the Palace, or take the metro if they were feeling lazy.