When I received my DX8 in early 2012 one of the cells in the NiMH pack overheated after few minutes of charging and luckily I had it placed next to me, so I discovered before any damage to the tx.
I received a new NiMH package on warranty, and that was ok however the voltage dropped quite quickly and made me feel a bit uncomfortable at the flying field.
Then I decided to change to LiPO and bought the cheap package from hobby king. It lasts for hours and hours of flying and I think I recharge every other month or so.
I would recommend using LiPO in stead of NiMH.
The disadvantage of the DX8 built-in charger is that it will not tell you how many mAhrs was charged, so a rapid decline in voltage does make it feel uncomfortable. The majority of model flyers have a computerised charger anyway, so Spektrum could save the cost of the built-in charger and lower the price or instead spend the money on developing and optimising the Airware.