my first extension took two visits and about 9 hours. If it was like that all the time I would definitely use an agent. My last one took two hours tops as I knew exactly what was required. My local office has been far more helpful since I got the stamp from them.
The other issue with agent obtained extensions is that your home immigration office will know and possibly make things difficult for you. That was the case for me though I did get most of what I wanted with a lot of patience and perseverance. I had to fly to Bangkok to et my stamp put into new passport. Bit of a pain.
In addition, on entry with my second agent obtained extension I was told in no uncertain terms to get my next one myself so I did. My local Iffuce has been much more helpful since. Just my experience. Get a visa your self if at all possible.
Nothing like a good laugh and plenty of those here in Thailand. If I hadn't had facebook advice from some very helpful people I would probably be in jail for overstay or some other thing I failed to understand. Good luck.