Buy it with Armored Princess if you're a newcomer. If you already have Armored Princess, wait for a price drop.
The game is $20, half the price of Armored Princess when it was released, so even though I was really looking forward to it, I wasn't expecting anything close to the huge amount of new content that was in Armored Princess. However, this expansion didn't even have a fifth of the new content that was in Armored Princess. In fact, a lot of the "new" content isn't new, it's recycled stuff from Armored Princess and the Legend.
This expansion mainly adds new features to the Armored Princess campaign. There are new creatures, artifacts, spells, skills, quests, and many of the old creatures have new abilities. Unfortunately, as I said before, many of these "new" features aren't that new. The new skills are just slightly changed old skills or skills brought back from the Legend. Most of the new spells are just the rage powers from the Legend. Only three spells are really new. The new creatures reuse the skins from old creatures.
If you've already played through the Armored Princess campaign then you'll mostly be treading old grounds. You can't load up your Armored Princess saves so you have to start over from scratch if you want access to the new features. This was the most annoying thing about the expansion. Imagine if DLC's for Fallout or Mass Effect required you to start a new game before getting access to the new content.
The two new mini-campaigns are simple boss rush maps that you can finish in an hour or two. They seem more like demos than campaigns. They add nothing that wasn't already in Armored Princess since most of the new artifacts, spells, creatures, and creature abilities are not available in the mini-campaigns.
The game does come with a game editor and there is definitely the potential for a lot of player created content but you do have to look at the game's fanbase. The message boards are pretty much empty. Even their official message boards are pretty scarcely populated. The game does not have many fan sites. Although I don't doubt that there will be fan made campaigns, it seems unlikely that there will be that many of them.
Overall, this expansion isn't terrible. However, it just doesn't add enough new content to justify the price. It pretty much adds what a DLC would add to other games except that you have to start over to gain access to the new content.
The one caveat is that if you haven't played Armored Princess yet, then it definitely would be worth getting Crossworlds with Armored Princess. There's a $10 discount when you buy Crossworlds with Armored Princess and that's a pretty good deal for people who haven't played Armored Princess since they can get the full experience without having to go through the tedium of replaying old content. On the other hand, if you've played through Armored Princess, especially if you've done it more than once, then the new content may not be enough to justify another playthrough.