Messaging is very straightforward. There is a shortcut on the Home screen that takes you straight into message creation, and from there you can add contacts and start to write your message. Or you simply choose a contact, such as one of the ones you've put on the Contacts Bar, and start to write.
Because the Nokia 6700 Slide doesn't have a touchscreen, you're reliant on the numberpad for text entry. It's flat and, while it is reasonably responsive, we'd have liked just a bit more from it. Flat numberpads never quite have the same feel as individual keys do.
You'll also have the usual problem with slider handsets that the top is rather heavier than the keypad section, so there could be a tendency for the phone to overbalance in smaller hands. It isn't as bad as it could be, just something to note and maybe try before you buy to see if it affects you.
There is a predictive text system which works well and allows for fast typing, or you can simply use the standard multi-tap system if that works better for you.
There's nothing really to pick at with regard to texting. It works, it works well, and quite frankly you'll probably find you can write texts faster on the Nokia 6700 Slide than you can on some smartphones with resistive touchscreens. There, we've said it. Knock us down if you dare.