Opinions and experiences with ORM / DAL tools

Hi all.

I am writing some paperwork about Object Relational Mapping / Data Access Layer development technnoligies and tools (like Microsoft ADO/DAO, LINQ to SQL, Entity Framework, (N)Hibernate, Doctrine, Apache iBatis and other SQL / CRUD generators and mappers). I would like to know other programmers' thoughts about the subject so my work is not based only on my own subjective opinion.

If someone here has experience with ORM / DAL tools (or have created own ORM / DAL) then I would be grateful to know about your findings. I can suggest some factors that you can take as a basis:

learning curve - how hard / easy it was to get into this technology, how long did it take; was the quality of the documentation appropriate?

introducing the technology into an existing project - how long did it take, was it hard, what problems did you encounter? Where there any problems - "deal breakers" that made you choose not to use some certain ORM ?DAL development tool?

starting a new project and choosing the ORM / DAL technology - how much the ORM ?DAL dictates the rules for an architecture of the new project? where there any compromises needed just to adapt your project architecture to the ORM / DAL technology?

porting some project to another data base / working environment - how did ORM / DAL tools help or create additional issues?

flexibility - if project specifications changed, how the ORM / DAL reacted? Was there any need to throw away existing ORM / DAL and choose another or create your own layer from scratch?

And considering all the above, what would be your ideal ORM / DAL tool, what features should it have and what should it avoid? Do you prefer high abstraction from SQL or maybe a tool that generates modifiable SQL / CRUD that you can tweak later and also the changes do not get lost when you use the tool to regenerate something?

And other useful ideas about this topic are welcome.

Thanks.