Welcome to Flyertalk, DSFFlyer.

Good review overall in terms of in-flight service, FF program benefits, or both?

Absolutely join some FF programs. Any mile not credited to a loyalty program is a mile of wasted opportunity toward status benefits and awards. "Only once a month" travel will easily put you on track for some level of elite benefits.

That said, lower tier elite benefits vary wildly from program to program. With all of them, you'll be able to use elite lines at check-in and security checkpoints (at airports that have them). Flying out of DTW you'll have a better chance of outbound upgrades on UA, for example, and will be able to pre-reserve Economy Plus seats regardless; on DL, many adv. purchace fares are ineligible for upgrades, period.

You'll do well by spending some time poking around the the various airline forums looking for info on elite benefits, esp. on the route(s) you're most likely to fly. Feel free to post specific questions at any time.