I've been using the ONAG for some time now. See these images with the ONAG + 2X Astrophysics barlow:
http://www.pbase.com/prejto/image/152783100
http://www.pbase.com/prejto/image/152287993
These were with my TEC140/PMX setup. I initially found guiding to be more difficult than I expected in the IR. I think this was due to several factors. 1. being a "beginner." 2. perhaps not enough light due to small aperture. 3. perhaps due to the way APOs deal with IR. 4. mount issues with my former MX.
Since getting my TEC180 guiding is a snap due to more energy in the IR reaching the guider. But, I am also not chasing any mount issues! Also, since I'm using an APO I must use an IR blocking filter (built into luminance or LP filter anyway) so losing the IR has made no difference to me as best I can tell.
One great advantage of the ONAG is that one has a HUGE field of stars to pick for a guide star and none of the problems faced by OAG solutions. The one drawback to the ONAG (other than BF) is the fairly crude guider focus arrangement. However, I have recently completely solved the issue after installed a custom made focuser made by Gary Jarrette. No flex, and perfectly reliable and EASY focus with no guide camera rotation. I have replaced the Borg helical focuser with this one as it is far superior. (see photo below. the focuser has c mount threads that go to t-threaded extension tubes)
Another most interesting development is the ONAG "Sharplock" focusing possibility. As soon as I have a TEC focuser solution I will be trying this as it sounds great. It is based on the change in shape of the guide star in IR as focus changes slightly. This shape change can be used to create an automated feedback to the electronic focuser. Theoretically it is possible to go for a whole night without leaving the target to focus. At the moment the software is a bit limiting such that you must use MaxIM or one other unknown package called PRISM. Hopefully TSX will shortly be included though I have no idea if such a solution is even being considered.
In conclusion I think it is a very good and well made system! If you have the BF (SCT or refractor) I personally think it may be better than OAG.
Peter