There are a number of ways you can do it and yes, steam does work well on the metal parts, but you do run a greater risk of melting some plastic parts or damaging electrical components it you blast the steam into the wrong places.
When I cleaned the engine bay in my brother's Civic, I used a pressure cleaner, along with different chemical cleaners and what not, however I wasn't to confident in using that on a Subaru engine as there's a few parts which are a lot more exposed.
What I did was;
Disconnect the battery and tightly wrap foil around any electrical components, or exposed parts of the engine/ connectors etc. which I did not want to get wet or get any product on.
Then I used a spay bottle of water to wet and dampen the areas I wanted to clean (that way you have a lot more control over the stream of water, and which parts you want wet)
I personally find it easier to work in sections - So I would spray down a section to wet it, then spray on an engine cleaner/ degreaser.. Leave this to sit/ dwell for a minute or so and then agitate it with a paint brush, tooth brush, rag - what ever worked best for the part I was cleaning - then follow up with another good spray of water and final wipe down with a clean rag.
Finally I dressed all the plastic and rubber parts with a plastic cleanser or protector and then waxed all the exposed painted metal parts.
I used some total auto cleaner stuff which I've purchased from specialty detailing places, but I have also heard very good reviews about Autoglym 'Engine & Machine Cleaner' which you can get at Autobarn (was at Autobarn Bankstown the other day and they have a good range of Autoglym products if anyone is interested..)
For the plastic and rubber areas, again I used some stuff which I bought off the net, but a natural look dressing for the interior or some sort of plastic/ rubber/ vinyl cleaner or protectant would work fine.
Not sure whether you Gen IV guys would remove the engine cover piece to detail, as I've never really had a good look as to what parts are exposed or covered in a Gen IV, but just be careful and don't drench any electrical or exposed mechanical parts.
Hope that helps a little!!
Cheers,
Jonno