Firstly I'd like to say, as with most graphical software, try before you buy! Their website allows you to DL a Demo version that gives you a fairly long time to decide before buying here at Steam. Some users have reported issues with Wacom tablets, however I've had no issue running it with a Wacom Bamboo.
ANYWAY, If you are familiar with Paint Tool SAI or GIMP/Photoshop you'll find elements from both in OC. It does lack the organic feel of SAI in a few ways but the functionality it has in other areas makes up for that. It has all the filters and adjustments that you'd expect from a tool like GIMP or PS, such as gradient maps, filters and blending modes. The brushes are a little dissapointing compared to more 'natural' paint programs but they offer both the pen style that users of older PS versions/GIMP will be used to as well as the more painty feel that comes from SAI. One thing to mention is they have a fairly wide range of tweaks for the pens and pen nibs so making something custom to suit your needs shouldn't be too hard.
There are a couple of things that make this program stand out such as the adjustible overlay perspective grids. More experienced doodlers won't find this too helpful but it is nice not to have to set up a grid each time you want to plan something out or check something. New artists will find this super useful.
Secondly the radial menu button is pretty neat, allowing you to skew your drawing, put it back as it was or zoom to fit page/snap to 100% easily. Lastly, possibly the most notable function is the ability to save your drawing as an event. Some people might feel this is gimmicky, but you can play back the process of a painting to see what went into it. You can stop or choose stages to pick up the artwork from too, so editing or trying a new direction is a lot more accessable with this. These can also be shared as files (fully lockable so people don't gank your art) between users too, which is a nice touch.
To sum it up, if you are looking for a cheaper art program that has fairly well rounded features this is a good choice. It has more convienience than other art programs out there in terms of design but you can't expect it to perform like an industry standard program. A good choice for hobbyists.