I agree with Jim, that many of the reviews for the plugins are lower than they rightfully should be due to expectations of the end user. For full disclosure, I will mention that I own a web development company that specializes in IDX integration and we have experience integrating all of these solutions on over 2000 sites.
With that said...I hope this helps.
The solutions mentioned from Diverse Solutions (dsIDXpress) and IDXBroker work well as does one not mentioned by iHomefinder called Optima Express. The dsIDXpress solution is not a complete IDX solution and has limited features, but is good for SEO. Therefore to be fully complete you would need to purchase their dsIDXpress solution ($29.95/mo) and their dsSearchAgent IDX solution (99.95/mo) for a total of $129.90/mo.
iHomefinder and IDXBroker both offer a plugin that allow you to display listings under your domain for the SEO benefit for less than $40/mo. Both of these companies offer solid, reliable solutions. Depending on what your goals are there are other options as well that go beyond the scope of this response.
If you are interested, as mentioned, this is what we specialize in. Consulting with our clients and developers and offering advice based on years of experience regarding which solution would most likely work best. We don't mark up the pricing for any service that you choose and you get the best of both worlds....the right product that meets your needs and an extra layer of support from developers that have already integrated these solutions. We can save you many hours of trial and error and in some cases code that will save you time. If there is something that doesn't quite meet your needs, we can offer you some advice on how to do it, quote to do it for you, or use our relationship with the IDX vendor to try and find a workaround.