We're not going to take sides here - the answer is that both open source and commercial search engines have benefits - and which one is "best" depends more on business requirements and staffing. We have worked with and written about both types of engines.
Business Goals that Align with Open Source
- Need to deeply embed search
- Need precise control over relevancy
- Unhindered Intellectual Property
The Not-So-Compelling Arguments
- It's free!
- It's been packaged into free Enterprise Apps like Solr, so now there's no coding!
- You have access to the source code!
The second list, while partially true, should not be the deciding business case for using open source - it makes much more sense when items from the first list are your dominant business drivers. For example, using open source search in an enterprise environment solely to save money is false economy; the extra staffing and implementation time will usually drown out any savings in license fees.
These are moving targets, these projects are getting better over time, and the Open Source community is trying to address many of the technical items.
Here's our current list of Free, Open Source and Low Cost Search Engines, and our list of higher end Commercial Search Vendors.