I have a DIY reactor, and I think it does a fairly good job for me. It handles the tanks needs which is my main concern.
Improvements on my design would to add a CO2 Re-Circulation line to mine, much as was discussed in the RC thread linked above. The re-circ line would draw from the highest point inside the canister, and return it as a venturi to right after the re-circ pump. This should help keep the pump from cavitating as the gas won't pass through the impellar, and should add some extra life to the re-circ pump. It would also help to increase CO2 efficiency, which is my reactor's main drawback. I have to run a fairly high bubble count to meet my tank's needs, due to the return line being located where the CO2 would naturally collect. This removes it from the canister instead of recycling it for another trip around the media.
On smaller tanks, this cost is negligible, but it could mean some $$ for larger setups (I only have 120 gal volume).
Just my $.02,
Cos