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Using an OAG is a cost effective solution over using a guide scope. Not to mention that a guide scope can and does get differential flexure. The main reason that people seem to have trouble with an OAG is getting it focused. It can be a fiddly process but worth the effort. Once you get the OAG focused with the main camera you should never have any trouble finding guide stars. I used an SSAG for years before it died and there were always at least 4 stars in the field. When I spent the money on a SBIG ST-i I found I would get triple that amount of guide stars. Orion now make a look alike to the later guide camera with the same sensor and it is much more affordable.
I have never used an ONAG but from what I hear those appear to be an excellent solution too, albeit one that is more expensive. One of the advantages of course is being able to guide in infrared. Not sure I will ever feel the need to go down that path myself but it would be useful if you have aweful seeing all the time.
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