Tax Liens in Maryland

This question is for anyone that can help, but looking for a response from @Ned Carey.  What are the pros and cons to buying tax liens especially the ones that haven't been sold in auction.  There seems to be quite a few that are sitting as low as $350, are these not worth the effort?  I understand you won't get much interest with 1 2/3% they publish but it seems like after 6 months the house can be yours.  Am I incorrect about this? And, what should you look for when finding a good buy when searching for tax liens?

Thanks in advance