Considering the sheer number of users that RunKeeper has, there must be a considerable crossover between people who love the app and people who want it to conform to Android's visual design style. For however many of those people that there are, today is a good day: the company announced a comprehensive Material Design overhaul for version 5.5.3. It's in the Play Store right now - no need to wait on a rollout.
Old on the left, new on the right.
Runkeeper was kind enough to compare some of the juicier changes to the app with the old version (a move very much appreciated by lazy tech bloggers!). In particular the Activities page has been given a huge change, with a new card layout showing integrated maps and more easy-to-read data. Most of the rest of the app gets a "flatter" interface and a few tweaks here and there, some of which will be welcome, some of which may not. The "start" page in particular features smaller text that might be harder to read outside or while running.
On top of all that, Runkeeper claims that the new version includes better animations (probably tapping into Lollipop's APIs on compatible devices), faster performance, and a bunch of fixed bugs.
- RunKeeper blog