Sorry, really late getting in on this one. I've never used one before, but have just got an Izettle. The reader is free, but you pay a higher transaction fee (currently as much as 2.75%). Most other readers have a lower charge, but cost money to buy (PayPal £40, Sum Up £70 etc.)
My main motivation for the izettle was the free reader. The 1% difference in charges between it and the Sum Up would take a while to make a difference (£7000 sales to recoup the cost) and at this stage every penny outlaid counts.
The izettle has some good analytics when you log in online and allows you to record cash sales through their App. Also it lets you take American Express, which some other don't (not that many use it).
Since getting it, I haven't been asked if I take cards (which is just typical), but it's nice to have the option.
Best wishes,
Damian