Just played through the Commander Lilith DLC with my girlfriend. First of all, it's only free until July 8th, and I know how lazy some of you bastards are so download it NOW. Some quick thoughts (spoiler free):
First and foremost it would REALLY be best if you have played Tales From The Borderlands before playing this. Otherwise some pretty major events from Tales will be spoiled for you (and these are major events in terms of the overall Borderlands universe). I highly suggest playing Tales (it's a really good game!) before this.
Second, the new loot tier "Rainbow" are awesome. Maybe it was just our playthrough but it was generous as fuck with them too. My girlfriend and I each got a grenade mod and SMG from random ammo drops, as well as some rainbow rarity skins and about 4 or 5 legendaries as well. And this is in a total playthrough of like 5 hours without doing a lot of side quests. There are also new heads and skins for you completionists out there.
As for how it is: it's good. A few surprisngly down to earth and emotional moments, a decent variety of new weapons and locations and the end boss fight was tough without being over the top ridiculous tough. It's worth paying for, so if you can get it for free do it.