The first in the 'Crossing Borders' series of articles by Bruce Tate has been published at developerWorks. It gives a good overview of the benefits of Rails' Active Record approach to persistence.
The second part of the article compares it with Java persistence engines and introduces RIFE as a promising alternative. RIFE's persistence engine hasn't received a lot of attention in the community and it's nice that it's slowly starting to get noticed. Combining it with meta programming through Constraints makes it extremely powerful and it's even able to handle content management and transforming transparently for you.