I haven't visited Howl O'Scream, but I did spend 3 nights at
HHN in October.
I found HHN amazing but some of the features were a little repetitive. There are a certain set of "scare actors" which roam around the park with a chain saw, setting it off at intervals to scare the guests. Whilst at first this does make you jump if they come up behind you and do it, and it is funny watching others reactions - after 3 nights of it I was ready to chuck all of these chain saws into the nearest recycling bin!!
Apart from that - I found HHN amazing. The horror mazes they feature are great - even if you don't get scared by them, going in there to see the detail is definitely worth your while. Even the lesser known themes for some of the houses I enjoyed merely for that reason.
3 major tips from me;
1) Don't just do 1 night. Even with an Express Pass, its too hard to enjoy everything to the extent you should be if you're visiting from the UK and won't be able to just pop back.
2) Get in the parks before regular park close. That way, you can be ahead of the huge queue of guests trying to enter just for the event. You can visit some attractions, and rest up at one of the lockdown areas whilst the Universal team set up the park for HHN. And then you can get on a couple of houses with virtually no queue.
3) Get an Express Pass for HHN. Its worth the money. I honestly would not visit HHN (being a visitor from the UK) without an express pass. I want to experience everything and enjoy it - rather than queueing for sometimes up to 240 minutes to walk through a house for 3 minutes.
I have some vlogs for my Trip Report which I'll be posting soon. Lots of HHN footage from last year which may help you out in deciding how long to go for!