Yarns can provide up to 4 channels of CV/Gate conversion in a variety of multitimbral and polyphonic ways. It also features some of the MIDI processing features of the Mutable Instruments MIDIpal, such as an arpeggiator, euclidean sequencer and an SH101 inspired sequencer! As well as this you also get clock dividers, a digital oscillator, portamento, swing, different tuning systems, 303 slides on the sequencer and so many features it was hard for me to decide what to show you.
All of the features are great to have in your system, in either a small/starter system or a large system I really think you couldnt go wrong with Yarns. All of the features on the module are navigated by the edit wheel, which at first seems daunting or scary, especially if you like your functions to have their own personal knob with which to coax as you see fit. But in all honesty, once you spend about half an hour with this thing you get to know the menus quickly and by the end of my time with yarns I was an acrobatic menu diver! It does become a little bit frustrating when working in multi timbral modes, however I dont think this is something that should dissuade you from buying it.
I think that £249 is a bargain for the feature set that you get. If you are just getting into eurorack and are thinking about a MIDI interface then this is the module for you!