The mobile browsers are mostly based off the same rendering engines as the latest desktop browsers, so in rendering fidelity they are pretty good if not excellent.
However, beyond the browser itself, mobiles in general leaves much to be desired in terms of usability due to their general lack of usable keyboard, inability to hover, need to constantly zoom in and out, usable-but-not-perfect text reflow, relatively slow processor and connection speeds, limited RAM, imprecise digitization coupled with a screen-blocking finger, etc.
That said, many websites, including most Google services, have mobile-specific versions for iOS and Android, with interfaces redesigned for touch-screen use. Those tend to do much better. Well-designed apps do even better, but they’re not always available for all the services you want, and sometimes even when they are, they’re crappier than the website (Facebook for Android, I’m looking at you.)
In other words, I think the mobile browsers are doing great considering how much they have to try and adapt to websites that weren’t designed for them.