Pros
Having worked at several REIs, I've seen that the sheer number of interesting, talented, and personable individuals the company hires creates the (often realized) potential for a fun and interesting workplace. The customers can be all over the place depending on where you work but you're well supported by management in dealing with difficulties. The better stores provide ample product and department training for their staff, even leading me to pick up new outdoor pursuits. Hours can be very flexible, but not always predictable. The discounts on gear are also phenomenal.
Cons
Management and management practices are a huge issue across REI. The company is following the industry trend towards making floor staff as expendable as possible through successive restructures. It's a progressive company only in branding at this point. Management practices increasingly reflect this - retaliatory strikes against workers who speak out against poor working conditions, lack of accountability, and general willingness to discard experienced and hard-working staff as they see fit. This is only buffered by managers with the personal integrity to counter this trend, although many store level managers are under heavy pressure from REI corporate.