Having seen a few places now, I have been told of 2 scenarios

a) the LA is just tenant grabbing, once we move in, the tenant deals with the landlord
the LA manages all the affairs, so we deal with the LA.

Should I be worried about (a) ?

I would be, but thats just me. Much rather deal with an organisation which has a reputation to protect than some fly by night Landlord.

But it's just luck at the end of the day, our last landlord was great our current (soon to be ex) landlord is a total arsehole.

Absolutely standard practice, either way.
We looked at many properties and asked the question. I prefer dealing with an agent so you can let loose when things are not right, he then dilutes when dealing with the LL.
I have a brilliant agent. Pure luck

When I was a landlord I was told the agent would do everything, and the tenant would deal with the agent. I would only pay the bills. This suited me as I was about to go and live 3000 miles away. But it was a complete lie. The agent did nothing. Once, the washing machine broke, and the agent let me know by sending me a letter with a single 2nd class stamp on it. It took three weeks for this letter to arrive at my new home in Hong Kong. I imagine the tenant hated me, but it really wasn't my fault.

Depends how you dealt with it though, if it was me as the landlord I would have rung the tenant immediately and explained the situation.

For me I think it really boils down to how you could deal with any problems if they arise, with a letting agent they will have some form of premises and normally some trade body you can go to (not that they ever do anything).

One way is to check out a few "let by" boards and go and knock on the door & ask the tenants what the agents are like. Some agents are notoriously awful and hated by the LLs as well, but particularly in smaller places they may have a virtual monopoly.