I really like the custom Blaster. In my opinion, it's a top tier threat on an organized team.
It's a great spacial control weapon and can 1-shot at close range with skilled shots. Point Sensor and Bubbler instantly make it a good team support weapon, as they are both extremely useful and cover entirely different needs.
Unfortunately it can get out-ranged by many weapons, and it has trouble dealing with more than one enemy at point blank range because firing with it slows the user.
Proper Custom Blaster use should revolve around playing a supportive roll, landing Point Sensors, denying the enemy access to choke points via intelligent spamming, saving teammates from trouble, and unleashing Bubblers at critical moments. Teammates will have to understand the Blaster's roll and when to position themselves in front of or behind the blaster. The user will have to know when to back down and when to keep spamming, which makes me feel that players well versed in zoning-heavy arena shooters such as Unreal Tournament will have a natural gnack for abusing the Custom Blaster.
I don't like the rapid blaster. I don't think it's very good at controlling space because its splash damage is low, and I don't think it's very good at killing because it can't one-shot. That being said, if the OP likes the Rapid Blaster, I think he should make it work at a competitive level, but if and only if he puts effort into learning the Custom Blaster first.