I have an '07 GTS250ie and after 22,500 miles I have had very little trouble with it. I've had to replace the drive belt twice and one wheel bearing. I haven't opened up the engine to do valve checks or anything that intrusive but most maintenance can be done at home in the garage. The internal packaging can be tight and I expect the 300 would be even tighter.
The steering geometry that makes the scoot agile at low speeds can make it feel very nervous on a gravel road. The small diameter wheels and limited suspension will probably make you keep the speeds under 40mph.
The cost of cosmetic damage to the steel body panels can be so high that insurance companies often will total the scoot after a low speed get off.