Well, I'm not an anarchist, I consider myself a kind of libertarian marxist. I have never found my ideal socialist organisation, as it doesn't exist, so I have linked up locally with whatever existing projects are going on and it is often important to have the back-up of some kinda national network to be an effective organisation. When I was in the SWP I thought that a massive upsurge of class struggle would transform the internal culture of the organisation, but who knows? I have much respect for the intellect and activism of many of my former comrades and have learnt much from them.
I have many disagreements with the SP, though to be honest it seems to me ridiculous that the SWP and SP are in separate organisations, they should be factions within a broader marxist organisation. I do believe that these divisions based on historic disagreements shouldn't dictate the present or the future of the left, as Marx said, "The traditions of dead generations weigh upon the brains of the living like a nightmare", I also believe in a greater flexibility of tactics.
As I've never been an activist in the SP I can't really comment on its internal culture, but from what I have heard from others it is quite top-heavy.
In my experience, some more libertarian forms of organising are deceptive, Consensus decision making, for example, while it is good to have consensus as a group and mutual respect, this process promotes conformity (people don't want to break the consensus and look confrontational by proposing an alternative view) and also means that a tiny minority can veto anything (within the Social Forum movement it has meant the most conservative reformists can dominate the process).
Many libertarian groups have 'no leaders' or 'hieararchy' this means that you either have to spend hours at meetings to be part of the process, or more generally that there are people who act as an informal executive who organise stuff between meetings and set up the meetings, but because they are not openly designated as officers of a campaign they are not democratically accountable.