Yep

. Not one bot registered since I've implemented it

. I have bout 20 different questions, no captcha. I have about 3 or 4 users active at all time and around 30 to 40 guests... I haven't checked but I'd bet at least half of them are bots... some just crawling, some trying to register.
Although, I have to add that the forum is not in English and that the questions are in Cyrillic, so... I guess that adds a little more extra security. But, hey, you can always just add Cyrillic or characters in some other language and do the questions with those characters included randomly in the strings, along with other Lain characters and ask users to write combinations of them in the answer filed (the first five, the last two and the first three, etc.), so even if the users don't actually know what the characters are, they do understand that they have to enter them in that particular order, which can be done with a simple copy/paste from the question

. This also might limit your user base, but... hey, I'm not making money off of mine, I'm doing it because I like it (and I love what the forum is about), so I'm actually not too keen on gaining users that can't get that the simplest way to enter characters which your current keyboard settings doesn't support, is through copy/paste

. I actually did a user purge about a month ago... out of almost 4000 users, about 2500 haven't posted anything and haven't logged in for the past 5 years.
If the forum is in English, I'd drop the captcha for a day or two, see what happens, if a few bots register, I'd either reinstate the captcha or add another question, see if that fixes it.