I don't think they are bad for beginners. They are pretty docile even though they get pretty big. Just keep a good tab on the humidity and temps and they'll be a joy to have. If you get a baby, you'll grow with the snake and the size won't be so intimidating.
Great snakes and ridiculously easy to feed compared to pall pythons. I go through all 4 of my boas in the time it takes me to get my single ball python to take the friggin feeder lol. Plus you have got to love the way boas look. They just look...better all around.
2007 0.1 Jungle het Stripe Kahl Albino BCI
2008 1.0 Kahl Albino het Stripe BCI
2008 1.0 DH Kahl Sunglow BCI
2008 0.1 Anery 66% het Kahl Albino BCI
2008 0.1 Normal BCI
2009 1.0 Salmon Pastel BCI
I think it's important to know the sex of any snake you own but especially in the case of BCI's. We have a male that is certainly not huge and easily manageable for a beginning keeper. On the other hand we have a 9 foot female that although she shows a wonderful temperment, well I would not call 9 feet at maturity something a beginner should perhaps be dealing with.
Boa's are bull strong so you need to deal with issues of caging security. They also have a tendency to have a longer strike range and a much harder strike than a ball python so that is a concern for an inexperienced keeper when it comes to feeding time.
I would suggest you speak to someone like Jimi who is very experienced with boas. This is a major committment and deserves lots of thought before purchasing. Also I would always strongly recommend you get your young snake from a good breeder rather than a pet store purchase.
We have a lot of ball pythons in our home but it would seem empty without a boa or two in it.