Hi Deb, interesting question...
I can only speak from my experience and what I've 'heard' from other VA's...but here goes
My own instinct has always been that any situation like this that requires a fee is something to steer clear of. That's just me. When I first got started I found one client for minimal pay and random hours (some weeks I worked a lot, others not at all). I clicked with her right away and things moved along nicely from there with viable referrals starting to come in about six months later; she's still with me two years later and still the majority (75%!) of my new clients come through her recommending me. That's cut out much of my need for marketing, and all of my need for marketing outside of my existing clientele.
With all that in mind, I can't pretend that mine was a textbook case for launching a VA practice.
I will say though, that working without a 'middleman' has been distinctly to my advantage (ie - I don't pay anyone for my access to work or for referrals, and I literally am the
solopreneur in my business - there's a huge difference between contracting your own clients, negotiating your own rates, etc. and relying on someone else to establish those relationships and details on your behalf.)
Now, not having had direct experience with these sites you've listed I can't honestly say that they are good or bad or otherwise - but the simple fact that their profit comes from either charging a fee for access to job possibilities or 'marking up your services' (or both) says a few things to me:
1) the clients are out there (it's just a matter of finding them/marketing to them/getting them to see that they need *your* service above and beyond anyone else's, and
2) they're willing to pay top dollar for experienced, efficient, [insert unlimited keywords for all the qualities of a professional VA] virtual assistance.
As long as those two things are true I just don't see that a service or agency is a necessity because there's no way that every potential client out there is going through these agencies.
If you really want to develop and expand your VA practice the best thing to do (in my opinion) is find that first client and work like crazy to impress them, show them you've got what it takes. Entrepreneurs and small business owners market themselves like crazy just like virtual assistants do - and as a result they have tons of contacts and business associates and they all share information and bounce ideas and questions off eachother. If you can settle yourself in with that first client and show what an asset you are your client is going to end up doing your marketing *for* you. It just takes some time (and hard work
)
I haven't even heard of two of the sites you listed, another I know in name only. But I have read many comments by VA's who work through teamdoubleclick and they all seem to quite like the situation - so that says something in that agency's favor for sure. For me, personally, it just wasn't something I wanted to do. I'd rather drive the car myself, if you know what I mean
If you're considering one of these directions I'd suggest asking for references from currently contracted service providers at each agency you're curious about before you go through their process, do a little research - the same as the agency is going to interview you...interview *them* and find out exactly how it works, how much of a markup they place on your services (and how much of that you get) and at which agency the VA's are truly happy to be involved and are receiving
ongoing benefit from participating. If they can't readily offer you that basic information then I'd be wary. Treat it the same way you would a potential client and evaluate things like communication style, amount of up front information provided, willingness to discuss with you in general - and find out how they handle taxes and the like, because the one thing about working as a virtual assistant is: you're an independent contractor; any situation that doesn't fall into that category makes you an employee - and you'd have to decide if that's a balance you're willing to cater to (if it's the case that you would no longer be truly independent).
Just curious, is there one or the other in particular that you're considering?
Looking for opinions
but I will get there. I'm still working fulltime and would like by the first of the year to be able to cut down to parttime hours there and dedicate more to my own business.
) - it will automatically get picked up by the search engine spiders, but especially if you are linked up from a site like this. 
Lily Chambers, The Virtual Office Goddess, LLC
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