authoritylabs was a ZR client at one time, and the founder decided to build his own. he had a freemium model and took a high volume/low cost approach with all rankings done in the cloud, and they have stumbled mightily along the way.

advanced web ranking (installed) is solid, but the SaaS business case is easy to make with enterprise vendors.

As for the others, the ones mentioned specifically do much more than just monitor rankings (in fact I've had discussions with them about powering their rankings), so we don't go head-to-head with them much. The smaller ones don't play in the enterprise space.

Many enterprise prospects are actually doing rankings themselves, and we can generally make a very strong SaaS/outsourced business case.