Mrs B has a Hainzmann Direct Drive system on her Helios. Its quite a high end kit and not cheap but it is good quality kit, I've had it fitted on three different bikes now.
I've also had some experience of the cheap Chinese kits that are sold on ebay. Though these are simpler bits of kit than the Hainzmann the two that we have had were both ok. One was fitted to a tandem which has since been donated to Charlotte's Tandems, the other was on my wife's Isla Bike but hasn't been used for years, I don't even think I have a battery for it any more.
Both of the cheap kits were fairly reliable is use, if I remember correctly over the 4 years or so they were in use one suffered a defective Battery Management Board and the other went through a couple of controllers. Though this can be an inconvenience the spares are cheap and readily available.
The Hainzmann been back for repair once however this was a fairly painless process as Hainzmanns man in the UK provides superb support. Aside from the support and build quality one key advantage of the Hainzmann over the cheap systems is the way it delivers power. Both the cheap kits were controlled via either a manual throttle or a crank rotation sensor, the hainzmann uses a bottom bracket torque sensor. The difference between this and a basic crank sensor is that it can deliver power proportional to rider input, rider works hard, motor responds, rider eases off, motor eases off. This makes for a much more 'bike' like experience in that you don't realise the motor is there (until you turn it off). The crank sensor systems by comparison feel much more on/off in their power delivery.