Two apps, with one goal: to enhance iOS messaging. Both BiteSMS and Messages+ let you quick-reply to a text without leaving the app you're in, or assign a gesture to compose a new message from anywhere in iOS.
BiteSMS has been around longer, and offers theming and tons of customization. Messages+ is more streamlined, and integrated into the stock Messages app. It's worth checking out both to find your favorite.
Typing on an iPad isn't ideal, but it's tolerable. Editing text, though, is a pain. SwipeSelection improves it dramatically: slide across the keyboard to move the cursor, and slide from either shift key to select text.
Tired of having multiple iMessage threads for the same person? Merge combines all of your conversations for each contact into a single thread.
It's another Cydia app that Apple would be wise to emulate.