Speech recognition works most of the time, but I often end up with one word that Siri just doesn't understand and then you end up spending more time trying to figure out how the device wants you to pronounce the word than it helps you saving time by using it instead of just typing it out. There are also some funny videos of foreign speakers trying to use Siri when it first came out - that has been exactly my experience with it as well.
Auto-correct is the other annoying one, which can wreak havoc when you're texting in multiple languages. Even if you manage to setup your device properly with all the different languages, it can still cause some problems whenever you mix the languages within a single message. However, the biggest problem I've experienced so far is when you send messages between parties where one of them doesn't have the foreign (latin based) language installed/configured yet. The replies I've received from people on their new phones often ranged from funny to cryptic, where you have no idea what they were trying to tell you. They just typed in the foreign text and hit send without checking only to have auto-correct send you something intangible.