I know Bitfinex from the very start last fall. Initially it was strikingly similar to another site we all know very well
But in the following months, the Bitfinex funder reworked significant parts of the platform and especially introduced this system of loans by users, which might help to put this notorious "bucket shop" accusation at a rest. The funder also managed to put together a team of people which overall seems to get along OKish. As a plus, the Bitfinex team was always very responsive, and helpful and quick with fixing bugs (and we beta testers witnessed the nailing out of quite a bunch of bugs). Bitfinex is aiming at operating legally, which will mean KYC and AML compliance soon.
You should note one thing though: the instrument offered by Bitfinex is called "Contract-for-Difference". This is a clever financial construction which has surprising low risk in
normal operation; it is
not a bet of "the house against its users" (please look up the details in wikipedia if interested). BUT, this instrument has one well-known fundamental risk, which can not be overcome with technology: A sudden and strong market collapse combined with an execution lag (yes, as we have seen it on Mt.Gox) can wipe out the whole business. This risk becomes smaller when the platform has additional reserves as a back up. The risk becomes smaller when the platform tries to tap into a larger pool of liquidity. But in any case, this
is a fundamental counterparty risk, and anyone using such a platfrom should be aware and limit exposure accordingly.