I just installed Conversation on my E71, and I like it a lot. The only issue is not being able to reach it without having to waste either a) having to waste a button on my six application shortcuts (home screen) or b) pushing my contacts one-touch key, then going to the dropdown and choosing conversation (then choosing the conversation I want to view).
It would be awesome if you have the option of replacing the inbox/sent in your messaging folder. This assumes that Conversation will handle all types of SMS/MMS media (I haven't needed to send an MMS so far).
Alternatively, in order to not lose the inbox/sent in the messaging folder you can make it a tab or option in the messaging section which in my opinion is the most appropriate place to put it. Making Conversation a separate program as opposed to a subsection of Messaging creates a clunky and inefficient UI.
If and when this suggestion is implemented, you'll have to make sure to update whichever software handles the one-touch key settings so that you can select conversation options when performing a one-touch key short or long press.