Or just dump all the point from Barter and get Strong Back instead?
Unless you somehow trying to carry 200 various grenade, I can't see Pack Rat being good.
First off, I had no idea what to expect from the game in terms of itemization, so Pack Rat seemed like an obvious choice for a hardcoe character. In retrospect, I could have gone either way.
For most of the game I had a 5 STR, so 200 weight limit. If I stripped down as much as I reasonably could, keeping 3-4 guns, armor, rad suit, some food/water, ammo, and a couple miscellaneous items (like the Caravan deck), I'd be around 80. I'd say about half of it was lightweight stuff that was affected by Pack Rat. Then, while exploring, often a huge percentage of what I'd gather would also be lighter-weight goods. Until the end of the game, I was constantly picking up any non-alcoholic drinks I saw (water, SS, cola), and sometimes alcoholic, all ammo, and any food that offered decent to good hunger reduction. There are also some weapons that are very light and thus affected by the perk.
Knowing what I know now, I could certainly have planned around Strong Back rather than Pack Rat. Or I could have just kept a companion the whole way; my FO3 experience made me take companions and set them to wait so I could sneak around alone, without realizing until I was 95% done that they're actually useful in this game. But, for all that, there were a number of occasions where Pack Rat helped more than Strong Back would have.