Kobo are excellent readers and give you much more control over the look and feel of the books by customising the fonts, pagination, margins and so on. Side-loading and library rentals are much easier than in Kindle and their PDF support is ‘better’ than Kindle, not brilliant but better.
They also tend to be a generation ahead of Kindle in terms of functionality, so if you want to know what Amazon will ship next year, look at this year’s Kobo.
Not to mention they have devices with 8” and 10” screens and pen input where Kindle have no obvious equivalents.