From a raw performance perspective, it's faster than Rails, but it has some issues with it's design that can cause extremely bloated memory footprints.
From a developer sense, it doesn't come with a lot (hardly any) of helpers, generators, or other fancy tools. So depending on your use case you could end up re-inventing some wheels.
It's biggest advantage is that it's small, lacks a lot of magic code, and doesn't force you into a specific structure. This makes it great for web services, small apps, or even small components of larger apps. But like anything else it's definitely not a silver-bullet.
By design it doesn't have a lot of helpers, generators, or other tools. Depending on your usage this could mean a lot of re-inventing the wheel.