While I'd rather that all miles never had an expiration date, I've been with United for more than 10 years and I'm used to the 18 month expiration rule. I don't fly UA much anymore, but I have a big balance with them, hoping one day to use them to SYD or similar trip. UA is pretty easy to keep the miles alive. Super Shuttle to/from the airport and get 50 UA miles per trip and you can also use Mileage Plus Dining.

Continential doesn't have as many earning options. Car rentals is one of the few, but, in my opinion and for my personal use, I'd rather have 1/2 to 1 Southwest credits rather than 500 CO/UA miles. At least under the current model, one Southwest credit is 1/16th of a free round trip. Figure a free round trip at about $200 and that makes 1 SWA RR worth 200 / 16 = $12.50. In comparison, 500 UA miles is 500 / 25000 = 2% of a free ticket. Even if you value a UA trip at slightly more, say $250, that is still only $250 x 2% = $5 of value.

With DL/NW and with TWA/AA before that, when the miles were merged, that was considered "activity" and thus reset the 18 month clock. I am hoping the same will be true for CO/UA.