The lower part of this trail is overwhelmingly deceptive. There is a decent parking area that is well enough removed from the highway, and the trail-head is usually in good shape. Unfortunately, the "trail" only goes about half-way up the hill. After crossing under the brush a few times, the trail kind of disappears and leaves the hiker with the options of following game-trails, forging their own, or just turning around.
The game trails (if you can find/follow them) are usually a relatively safe option, but will quite often involve some talus-slope scrambling. It is usually easier on the ascent to just climb up to a ridge and follow it up to the summit rather than staying in the canyon itself. Of course, the decent is more of the same nastiness of sorting out the "easiest" path back down through the talus and brush.
The views from the top are excellent, and worth the effort on a clear day. And the overall distance and grade of the hike is great training for higher elevation mountains (it is a similar profile to the climb from Paradise-to-Muir on Rainier.
If you have an adventurous dog who will go anywhere, this is a good challenging hike to wear them out. If your dog has soft/sensitive feet, is prone to getting brush stuck in their fur, or is nervous scrambling on loose rock, it would be better to leave them at home.
Bring plenty of water. Although the name is "Waterworks Canyon," the water in the canyon can be quite sparse, and almost non-existent on the upper part of the hike. You can filter water from the stream, but only if it was water in it (not likely in late summer)!
Be Prepared for ticks and rattlesnakes. Both are quite common along the trail.