It's a very effective tool when used correctly, you need to make sure to let it break in and to direct the outlet of the reactor to the inlet of the skimmer.
The downsides I can think of compared to other carbon sources are:
1. Requires a special reactor.
2. Takes quite a few weeks to break in.
3. Hard to adjust the flow to be just right for proper usage.
4. Could easily bottom out nitrates if not careful.
The reasons to use one can be many:
1. You want to reduce nitrates
2. You want to stabilize fluctuations in nitrate by removing slightly more than you need and dosing it back again for fine tuning.
3. You want to reduce phosphates, but your nitrates are limited by carbon.
I've been using it for all of the above reasons, for years, and found that running it in an a well established system can one really benefit from it, more than a freshly setup system however.