The learning curve is relative to your experience in ETL work...
Using Spoon UI Designer in PDI (Pentaho Data Integration) is relatively easy to 'draw out' a transformation, but you do need to have an understanding of the 'ETL Craft'.
No ETL tool can hand-hold you through the best ETL practices, however some tools can give you feedback of your decisions earlier on in a test environment before going to production. PDI/Spoon UI provides different profile metrics as far as records/sec on a per step basis, which makes it easier to see your entire 'flow of data' and find any steps that may be hurting your performance.
The 'ETL Craft' is a caveat I put in before saying the tool itself, if you know ETL, is relatively easy to use but still gives you the power/control desired - you just have to make sure you make good decisions! :-)
I think the learning curve of Pentaho in general and Kettle in particular depends on:
- your previous experience with BI tools
- the resources you have handy.
With regard to the first item, nothing can be done to accelerate the process. But with regard to the resources, if you browse the wiki, buy some Pentaho books, and look for help and advice in the quite active Pentaho forum, you will be able to learn the basics in no time.