@clive 82 I've only had her for the day, so I haven't gotten to know her yet. However, I will be rehousing her tomorrow (she has outgrown the deli cup she lives in), so I'll see!
Supposedly Ceratogyrus marshalli is moderately defensive for the genus but a good first Old World tarantula, as it is relatively manageable for baboons and not a pet hole. (And one of the reasons I chose it was because I could get a confirmed female just by looking for the horn.)
@clive 82 I don't know about other members of the genus, but @cold blood said that only female Ceratogyrus marshalli have horns, and they start to develop a visible button around 2".
For the limited interaction I've had with her so far (giving her a mealworm and taking a quick photo) she has been easygoing, but I know not to assume she will always be that way, especially when it's time to try to get her out of her home and into a new one.
I'm hoping the rehousing will go smoothly, as it's not my first rodeo (even though it is my first OW). I think she will be a lot more comfortable in the new enclosure, which will have vertical space for burrowing and a hide. (The deli cup she has been living in -- I can tell she has because it has substantial webbing -- is too small, has no hide, and no place to dig.)