For those of us that came into backpacking early-on (in my case early/mid 1970's), REI was one of the only options. Great service, decent prices, and a place you could actually order gear from.
REI definitely lost its way in the mid 1980's and into the 1990's, but today they're own branded stuff competes with the best out there. Something that definitely couldn't be said in the 70's, and many years after.
I paid $5 for my membership in 1975 and have never regretted it... might not be de-riguer these days, but it's still a pretty good outfit, even in an internet age of bored, insufferably unimpressed, gear heads...