I can't say I go out of my way to study agents before viewings, but I actively avoid: Yopa, Purplebricks, Strike, The Square Room. I know that once they've had offers, their customer service falls away to nothing, and they're not capable of holding a chain together if there are problems (in most instances).
There's a few in my area that try to push really shitty properties on us, ones where sales have just fallen through and they're desperate, who whilst having a stranglehold on the market, don't have enough agents to service the numbers on their books. I don't want to wait five days for a viewing, that's idiotic, by the time you make time for me, I've already seen another three and am quite likely to offer! Also, they just don't GOSH. 'Can we do Saturday at 2pm.' 'Sorry, we're conducting, we're full, how about Weds at 3pm?' 'No we can't do midweek, ever, how about next Saturday' 'No, sorry, can't.'
So the answer is, no you cannot see this house that you may want to buy rather than 'we'll ask the vendor if they can make an exception.'
Shitty agents like that are why people get friends to mystery shop looking at houses to see what agents say.
It's interesting what @WorkCleanRepeat says about self employed agents. I'd classify those a bit differently to bricks and mortar high street agents starting up in this economy. They're probably ones with the balls to jump the burning ship and may have excellent experience and contacts.
I have no idea how you go about finding an indie though!