I used Flex/Bison for writing my first interpreter, and I definitely recommended for beginners. I think when you're starting out, it's more important to focus on writing a proper grammar and traversing the AST than to do all the lexing/parsing by hand. Plus, it makes it easier to rapidly prototype your language when you're still in the design phase, and the Flex/Bison files serve as a form of documentation.

That said, now that my language's grammar is stable and I have a better understanding of lexing and parsing, I'm thinking about ditching Flex/Bison, mainly to achieve more descriptive error messages.