There was a local grants scheme in operation in our area, which means that we and many of our neighbours have them. Basically when they are good, they are very, very good. But when they are bad, they are horrid.
If it's a decent (e.g. German or Austrian) make, and well installed then life is generally good. All that's needed is to keep it serviced and to empty the ash bin once in a blue moon.
Problems people have encountered include blockages in the conveying pipes (usually bad design or layout), dusty pellets choking everything up (buy the more expensive ones), and damp pellets sticking to surfaces (keep everything very dry). Apart from that, life is good!
Edit: Find out if it was installed under the Renewable Heat Incentive (RHI). There are / were two schemes - domestic and commercial. The domestic scheme for instance provides a tasty flat rate payment every quarter based on the size of the boiler installed. This runs for seven years, and I'm sure that if you buy the house, the entitlement (and responsibilities) under the RHI scheme will transfer to you for the remainder of the term.