We use our United Visa for everything, like many who have posted, and pay it off each month.
DH is also able to use it for all his business expenses, he travels A LOT!!, then his co. cuts him a ck.
By doing this we are able to accrue about 2-4 thousand miles a month, plus double mile specials.
United even has offers if you eat a certain restuarants they give you extra miles...I got 100 points for a $16 dinner!!!
We bought 3 ff tickets for our upcomming wdw trip dh, 2dd's but we were 12,000 miles from another ticket and I figured there's no way we can get that many miles in 3 months, so I was able to get a ticket for myself on the same flight for $450 (from west coast), after putting all our vacation on the visa I now have just shy of 25,000......DH is saying "I told you to wait" but I just figure we now have one ticket toward our trip next year

so now I'm a year ahead!!!
We would never be able to afford to fly without the ff tickets.
I would highly recommend it.