I've used Kettle, then PDI :-), for many years as well.
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The strengths compared to previous ETL solutions most companies have in place (i.e. DTS/SSIS is the most common I've seen if you have MS SQL installed anywhere) includes:
*Strong declarative approach to ETL design.
*Database agnostic approach - don't have to worry about a particular ETL solution working 'great' for one database, and poorly for others.
*JDBC driver access - This is gonna sound odd to the non-JDBC users, but ODBC and ADO.NET providers I keep running into sporadic/unusual issues in driver configuration/server configuration/something else unknown on many different database setups.
JDBC has been consistent and reliable, which for ETL is very important. Yes, someone will inevitabely say you loose some performance, well, loosing 1%-5% performance for rock solid reliability is an easy sell for me.
*Built-in warehousing support (dimensions), included, free, in the open source version.
*Customize/create your transformation step using
Java (SSIS you can do this with .NET as well). I've created an X12-style EDI parser in about two weeks to solve a particular business need (it was very specific and not contributed unfortunately).
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Compared to other open source ETL solutions (talend, clover, and several others I've reviewed in the past).
*LGPL license. You can use it in your business without worry.
*Commercial support. You can use it in your business without worry (Talend has this as well).
*Full-featured (Talend has this as well).
*It's got Matt Casters! :-)