This is one of my favorite spots in the Phoenix area.
I'm fortunate enough to be able to see it on mornings that I take the light rail to work, but the place really shines at night. In cooler months (note: Phoenicians typically consider anything 90 degrees or below as "fall weather), it's an awesome place for a late afternoon/early evening run or just walk in the park. You'll see runners, families with their kids, and ASU students out enjoying the weather. Great to people watch, or, if you're feeling up to it, renting a kayak (I haven't tried it yet but hope to soon).
My favorite spot at night is the dam on the west end of the lake, over by the performing arts center. It's a spectacular view as the sun is setting and the area lights up once it gets dark. Perfect spot if you're looking for somewhere to take your date.
This is a pretty popular location for events. Oktoberfest takes place here every year and I'm told it's quite the scene.
Downside: Be aware of your surroundings once it gets dark. It's a nice time to enjoy the view and weather, and the times I've been there have been plenty of other people around, but homelessness is a big problem in Tempe and this is one of the areas they tend to congregate in.