What are the weaknesses of open source BI?
I think the biggest weakness is that open source BI software doesn't always have the best maintenance & support to back them up. If your needs are pretty basic, though, you can't beat the fact that open source options are free and easy to implement.
What are some of the BI solutions you feel are worth a look for those considering an open source solution?
I believe open source software is generally recognized as being more difficult to implement (that is, if we're talking about free community editions). Usually open source BI deployments require users to possess software development skills. Not a big deal if you have an IT department, but if you're a small shop you may find free open source to be more trouble than it's worth.
Many open source systems are powered by Hadoop, which is handy, but a bit of a pain to get to work with a lot of the larger BI programs such as Cognos.