Is a laptop cooler really effective and useful?

Yes, a laptop cooler can help. Running hot can lead to performance problems and shorter equipment life. I don't have first hand experience with heat problems in an actual laptop (yet). An older laptop I have does tend to run very hot (too hot to touch on the underside) if I don't use a cooler, but with the cooler it stays reasonably cool.

The most recent example of equipment problems due to excessive heat I've seen was in a wireless router I have. A few months ago some stuff was put next to it, blocking its ventilation. It started having problems - frequent drop outs, for example. When I checked it out, I found it was extremely hot to the touch. I moved it to a place where it cannot be surrounded with stuff and it has operated without problem since.

A side benefit is that the laptop cooler I have sits under my laptop and allows me to change the angle of the system. This makes for better ergonomics. (Already having eye problems and CTS, I appreciate that.)

Yes, but the effectiveness varies by the design of the cooler. See this review of the Cooler Master NotePal U3 at Overclockers Club for a comparison of laptop coolers; all of them yielded lower temperatures than a laptop being placed directly on a desk.