The only issue I've had with Concentration is the minimum DC when hit. A single point of damage has a 45% chance (per-modifiers) of causing the spell to be lost. I felt either the minimum DC should be 5 or there should be a minimum amount of damage to cause a concentration check (I would say 5 points in a single turn).
Even with this issue, our group still uses Concentration spells regularly, because they're very strong. We just make sure not to prepare too many spells with concentration (about 1/4 seems right). Another thing to be aware of is to not overlap the use of those spells (offensive, defensive, or utility) since you can't use more than one at a time.
Even with this issue, our group still uses Concentration spells regularly, because they're very strong. We just make sure not to prepare too many spells with concentration (about 1/4 seems right). Another thing to be aware of is to not overlap the use of those spells (offensive, defensive, or utility) since you can't use more than one at a time.