Their seminary upbringing is very similar to FSSP and so they produce the same great product, a wonderful confessor that you can’t find anywhere else and makes you think you’ve been transported back in time 100 years (thats how I always feel after confession with FSSP and ICK). We belong to an FSSP parish in Nebraska but the air force has us out here in California for a year and so we drive a good ways to the nearest ICK parish. We feel right at home there. I think the only difference in their seminaries is that at FSSP they do Aquinas and Alphonsus Liguori, while at ICK they do Aquinas and De Sales as primary focuses. Also, ICK is significantly smaller as I understand it.
Bottom line, you’ll get masses, confessions, and homilies so darn Catholic that you’ll be scratching your head wondering why the rest of the church isn’t this way, just as you would at an FSSP parish.
Anyway, sorry for the rant, I’m just very grateful to have found these guys and can’t say enough good about them.
EDIT: Oh, and the ICK have sisters too, FSSP doesn’t.
Edit: I’ve also heard that an FSSP priest once said the ICK were a bit more intellectually/academically elite compared to the FSSP which is apparently more pastoral. Though I haven’t noticed a huge difference in either of them.