I'd heard that TopLink was the product of choice for object-relational mappings.  
Java Report described it as  "A rock-solid, mature product that fits a lot of needs. 
The better you understand the problems of object-to-relational mapping, the more 
you appreciate how well TOPLink addresses those problems."
So I was quite interested in doing some work for a company that was trying to 
standardize on TopLink.
One of the staff was showing me how to import a database table through the 
TopLink GUI tool for establishing mappings between database tables and JAVA classes.  
He clicked OK and suggested it would take 5-10 minutes to complete.  
"Wait a second," I said, "it can't possibly take more than a couple
of seconds to suck all the metadata from an Oracle database, let alone 
for a single table!", but his estimate turned out to be about right, 
and in line with my own later experience.  
When taking that long, you'd expect the results to be beautiful, but it 
would generally screw up if there'd been a change in the database effecting a 
primary key, that change wouldn't be reflected automatically.
The TopLink GUI tool allowed mapping either JAVA class public data members 
directly or else through get and set methods.  
Oddly, though, even if get and set methods were being used, there still had 
to be an identification of a single data member even when the accessor methods 
did not correspond to a single data member.  
In some cases dummy variables had been added to a class to serve this purpose.
Staff seemed to be taking an inordinate amount of time to solve simple mapping 
problems involving three table joins, and there seemed to be a lot of problems 
handling inheritance, but I can't comment on these further since I only implemented 
simple mappings myself.
One thing that was noticed was that TopLink applications were using up all the the 
available Oracle database connections.  
It was pooling database connections (which is good) but by default it was opening up 
six connections (even if it only ever used one, which is bad).  
This was reconfigurable with some work - just that the default behaviour was bad.
Another thing was that by default TopLink was caching everything, which I found a 
little strange; the database is after all a shared resource, which is the whole point.  
It took a second or two to figure out why a change made in the database was not being 
reflected in a read. 
Again, this can be turned off with some work, but it's an odd thing thing to have as a default.
It's OK to have for static data, but not generally.  
I think one of the reasons for all the caching was to achieve reasonable performance and
still access data in a way that favours many reads over a single join.
I used the on line documentation a lot trying to find ways of solving problems.  
There are many ways of doing the same thing, and many classes share the same interface even if a 
good part of that interface is not usable in a given context - TopLink's own class hierarchies did 
not seem to be particularly well designed.  
A problem with the documentation was that there user documentation did not show import statements in examples or give any
information about what packages classes belonged to.  For that you had to
go the reference documentation, and hunt through what were a fair number of candidate packages.
More generally, I don't think you should try and map classes directly with tools like TopLink, but rather map proxy classes and 
have a factory method to create the real object.  
If you don't do this you'll end up compromizing your class design too much.
Anyway, I don't think I would recommend this product.
Regards,
Daniel Parker danielp@no_spam.nshima.com