I've had status on both United and Continental for more than twenty consecutive years. I think I have a feel for both of them.
United is my primary carrier for two reasons: A) Ease of upgrading on international flights with SWUs and to some extent, by using miles. And B) Economy Plus.
Compared to UA, upgrading international flights on Continental is such a nightmare, that I consider it impossible.
If the merger goes through in a way that the CO's upgrade system and/or the OnePass program prevails, I would probably take all my business elsewhere.
I am not pleased about the possibility of this merger. The most likely scenarios hold very few positives from my view.