As a basic email client, Metro's Mail app is fine, with support for most popular account types (but not POP, the most common account type -- shocking!), rich text formatting, and contacts integration. Windows 8 and RT devices' wider screens let Mail show your folders, your message list, and the current mail message simultaneously.
But Metro Mail takes more work to use, and it's often unintuitive. For example, to search your messages, you have to use the Search charm, and you get no filters such as From or Subject. It also lacks the iPad's handy unified inbox view, message threading, and ability to have separate signatures for each email account. Additionally, connecting to Exchange from outside the office can be problematic.