Hmmm...I may have to look into the Carhartts. I've recently switched from jeans to Duluth Trading 'firehose' pants. Both flex and regular. I'm pretty happy with them; they seem to hold up well to diesel, oil, bilge grime, welding and grinding, etc and they shed water pretty well to boot. I had been destroying a pair of jeans every other month or so, and no holes in these yet. They are a little more than the Carhartts, but if you get on their email list they seem to have sales pretty regularly.
http://www.duluthtrading.com/store/mens/mens-pants/work-pants-for-men/work-pants-for-men.aspx
Can't say enough about the DTC Firehose/Flex pants. Father-in-law (who swears by their long t-shirts), gave me a gift card a couple years ago, I bought a pair of the Firehose pants, and haven't bought anything else since. Currently have the 1 pair of 2-year-old Firehose and 3 pairs of Firehose Flex pants. They're nigh indestructible, uber comfortable - even for larger guys like me, a 42x32 - and have all kinds of nifty little features that spoil you.
For the most part, mine still look good as new, though the darker colors tend to show their age more. I left a pair of networking scissors in a pocket of one pair which snipped a tiny hole, and my first pair of Flex finally started to show signs of wear in one knee after about two years. Sent them back to DTC and got 2 brand new pairs in the mail - free of charge, no BS warranty - inside of two weeks.
Recently tried out a pair of Cabella's hiking pants (non-ripstop). They're comfortable, but nowhere near as useful or fitting.
Can't recommend DTC enough. The regular Firehose pants are great for colder climates, as they're very heavy and thick - more than double the weight of a pair of jeans, I'd guess - but the Flex versions are rock solid. Buy with confidence.