Hmmm ... I am inspired to respond to bulldozer.
I use IB and am happy with them. I think that there are also other good companies out there but lots of bad companies as well. The web chat based customer service is good unless you ask either too many dingbat questions or very very hard questions. This is a lean mean efficient operation and they do have some difficulty with either extreme.
Now, on to bulldozer's points (apt name):
- no one charges margin. margin is just an allocation of capital required to cover the risk on your position. if you are up to your margin when trading a normal strategy YOU ARE OVERTRADING
- if you are getting margin calls forget about how many days grace .... this is a huge warning that something is VERY WRONG ... figure out what you are doing wrong before you bust the account.
"I would rather pay high commissions with a broker that charge low or none at all marg above the clearing house and also give a few days grace to pay marg-calls. One broker once closed my written legs and left my long legs open because the margin calls was paid just 6 hrs late."
Again, margin is not a charge, its holding money to cover risk and if one is running into this space I suspect one is doomed. I note that "one broker" wasn't IB but if I was a broker with a customer overstepping his risk I would close his positions too.
"Ask them how do they manage to stay in business charging peanuts on comms. Remember high marg means LESS trades and less trades means LESS profits"
They have been in business for quite a while and provided a better deal than most, much of that time. They do it by having volume, good electronic execution, and not too many support people. They are lean and mean.
Adequate margins mean that the traders are trading within reasonable risk limits and hopefully protect some people from themselves. I think most brokers would rather gain their profits from customers who trade sensibly and are able to stick with it for long periods ... low margins > customers going broke through overtrading > have to get another customer (and have a dissatisfied losing customer while you are at it).
I think that you make choices of:
cheap, fast, quality, features etc.
If you want high commissions and high service then talk to someone like fox futures or attain capital but if you are NOT A DINGBAT and
you want fast, good features, and cheap commission then IB is a great choice for trading the electronic markets.
Note: My comments about margin relate most strongly to directional stock or futures traders. I think that bulldozer is an options trader and perhaps controls his risk by other means. In that case a more expensive specialist options broker who (maybe, better) took into account the margin effects of options strategies would suit better than IB. He may well not be a dingbat. 
