How good is a Body Slam Lax?

I don’t know if it’s better or worse than Heavy Slam, but since it gets STAB and has a better chance of getting weather boosted (at least in my area) then it is probably better. Plus, it’s a three bar charge move on defense, way more likely to land a hit.
Fighting Snorlax with Body Slam in the old gym system was pretty grueling but now because of gym decay... it doesn’t really matter. Just keep it under 2400 CP and you’re good.

It's better than heavy slam hands down. It's up to you if you want to power it up. I only see game value for it in active area where CP decay doesn't play too much of a role.

I would power up a BS Snorlax regardless, but that's a matter of taste. Got a 3000+ CP Hybetbeam and Heavy Slam Snorlax that I out into gyms if Blissey is already occupied.

It's more threatening then HS because HS is meant to counter Tyranitar (and to a lesser extent, Gardevoir), and a Blissey with DG in the gym can already cover that. You can make the case for HS in a gym that doesn't already have Blissey in it, but in the case why not just drop a Blissey in?

Obviously defenders should not be a priority over attackers but if you have the resources and live in an area where 3k+ defenders are good, then your BS Snorlax is absolutely worth powering up. It will perform better than a higher IV HS Snorlax, moveset matters more than IV.

37/45 is already a wonder. That Snorlax is definitely worth using as a defender and (if you live in a high-turnover area) powering up.

IMO, it's the best defender in the game after Blissey. I value my defenders by annoyance factor and resources required to defeat. BS Snorlax meets both of those criteria.

If you're going to take my gym, I'll take my piece of flesh.

I've got two BS Snorlax (and one HS) over 3K. Great little gym guardians.

Using Tyranitar instead of Machamp will almost double the time required to beat Snorlax. I know its doesn't feel that long but if you use Tyranitar against Blissey you will realize that Machamp is much faster.

In addition:
BS is faster(increases the chance of hitting the attacker)
BS is 3 bar move (increases the chance of hitting the attacker)
BS has permanent STAB regardless of match up
BS will make the attacker dodge more and therefore increase the time to beat snorlax.

Max out defenders is only worth using in a busy gym