I have the HV-Maxx kit (the older 4400 version) mounted in my e-maxx/t-maxx hybrid and like it a lot.
No problems with the kit itself at all. Since I used my t-maxx front and rear ends (the t-maxx uses beefier diff housings than the e-maxx) with Maximizer cups, RRP gears, MIP CVD's all around, RRP slipper with steel spur, steel idler gears in the tranny and pinion, the drivetrain easily takes the strain the brushless motor puts on it. I actually prefer to it instead of one of my nitros due to the ease of use. The instant torque beats all nitros I have come across off the line easily.
I plan on upgrading the motor to the the new 6.5 by spring.
I recommend the kit to anyone looking to make the switch to brushless. They just have to be aware that upgrading the drivetrain is a must.