Stability is good in 9.2.1 - make sure you are patched up of course.
Solaris Sparc is very stable for both 32 and 64bit.
Solaris X86 running on VMware looks good but only been running for a few
months so not yet convinced.
I've got zero down time after 200 days on Win2008 Sr2 64bit.
Generally 100% uptime on Linux except where the boot drive failed.

Yes faster - about 10-20% for some queries.
Better support for .net connectivity.
Very little pain on migration - I've moved around 20 databases.
Clean installation process.
9.2.1 replication via log shipping is much more reliable than earlier
versions.

The only gotcha I think worth noting was mixing 9.2 and pre 9.1 Ingres Net
clients. I found an inconsistency with dates being written to the DB.

Paul