I just ordered this 4 channel charger for $20. Is it decent?

Here it is.

I was using the stock Eneloop charger, and came across this one when ordering some coin cells for my son's train set. It seems good, but I'm a little concerned that charging the cells so rapidly (150 min) could damage them. Is this a valid concern?

Damage may not be the right word. The worst it could do is mainly shorten their life by a few cycles. It's probably pretty good. Just keep an eye on them while charging to ensure they don't get too hot. Some of these "quick" chargers have the tendancy to fry your cells, and if you tend to charge more than 2 at a time, they don't have much breathing room. Like most, it claims to have a temp sensor and should shut down if the action gets too hot! A more expensive unit will give you the option of charging at various degrees of mAh. There are many schools of thought on that, with some saying slower is better, and others claiming the opposite. On the Lacrosse BC-700 for example, the "default" charge is only 200mA which can take up to 15hrs to charge 4 cells! Obviously this one you got, is going to throw alot more than that into the cells.

Taking two hours to charge a battery isn't a very high rate, especially since it is monitoring temperature. This looks like a pretty good charger to me.

Thanks all for the reassurance.