Sonic boom isn't a very good skill, I however, have it maxed. Like you, I don't use Wither or Leech, because I find their damage too low and DOT isn't very good in shipyard on the easier levels, and the actual damage output of these skills isn't very good regardless. I use pain quite often and have it maxed. I use it on the cannon mobs because their knockback is SO ANNOYING (Artstar and I have a mutual hate for this knockback ) The 3 second stun is very helpful, and can often (when combined with other classes stuns) kill these annoying enemies off without them hitting you. Sonic boom is good for causing small amounts of damage to the whole group, and even though the damage is minimal, area damage is still nice. I find engineers don't have the best skills and are often left with extra points after configuring to their play style. I personally dropped protection completely for a more damage based build (and find +60 armor to not be a big help at this point). I also put my extra 2 points at the end into revive (only lvl 35 on my eng) because the extra range and lowered mana cost help.