The Twin otter is the best designed aircraft I have flown when operating in the role that it is designed to do. It does its job very very well, with minimum fuss. As long as you arent trying to make it do a job outside its envelope, it "should" make you money.
I never had the luxury of anything bigger than the -200, so the old PT6-20 was what I operated with for 1600 hours. Great, simple engine, but a little underpowered at max weight especially on one. Another downside to this engine was finding engineers that knew how to coax the best out of 50's technology.
Some downsides.....
I found the forward and aft cargo areas very roomy, however the size of the doors limited the size of freight that you could carry. Many times I cursed the designer for not making the holes bigger.
The large beanbag like tyres were a bugger to balance (static balance - no worries, but dynamic balancing was the problem), so had a number of problems with nosewheel shimmy.
The lack of decent airflow when operating in the tropics was a problem, and a huge dislike of regular pax. Being Canadian, it was no problem to heat the inside, but cooling it without aircon was almost impossible with a full load of people.
Some upsides.....
Very high performance aircraft. Not in a speed sense, but in where you can put it down! Absolutely incredible, even with just using Flap 20 for landing. Not as good in a short field landing contest as the Nomad, but would rather be in the otter when trying to get out of a dodgy strip.
Can use it single crew or multi crew, depending on what rules you operate under.
Still my favourite machine.