What a bucket full of rubbish. I've dealt with the DLT 3 times since being here and immigration for 10 years. Never have I used an agent and never have I had any issues whatsoever. In fact I have always found them to be helpful and reasonable.
I rather think that it is ones own attitude that produces ones opinion. Bad attitude results in bad impression.
And for your information, neither of the above entities are 'government agencies' as you called them, they are government departments. Get it right!