Re: the lac operon - how useful is it as a gene model in all organisms?

The lac operon and the trp operon, both from the bacterium E. coli,
are the two most commonly used examples in teaching gene regulation in
prokaryotes.  They have a lot in common with operons in a wide variety of
prokaryotes.  If you go to the NCBI (National Center for Biotechnology
Information) Entrez query page
and type in "operon", you'll find over 7000 hits to nucleotide sequences of
operons in various prokaryotes, as well as over 22,000 hits to literature
citations involving operons in PubMed.  Almost all of these will be operons
in prokaryotes (bacteria).  

This is because, generally, eukaryotic mRNAs contain information from only
one gene, encoding one protein.  Prokaryotic mRNAs are often polycistronic,
that is, they encode multiple proteins -- like the product of the lac
operon when it is transcribed.  Polycistronic mRNAs do occur in eukaryotes,
though, (see T. Blumenthal, Brief Funct Genomic Proteomic. 2004
Nov;3(3):199-211 for a short review).  

The lac operon model has broader applicability, though.  The basic idea,
that trans-acting proteins can bind to cis-acting sites in the DNA and
either reduce (negative control) or increase (positive control) rates of
transcription, is applicable to all known organisms.  Both of these types
of control occur in the lac operon.  The CAP protein binds to a site just
upstream of the promoter, and when bound, it assists binding of RNA
polymerase -- that's positive control.  The lac repressor binds to an
operator just downstream from the promoter, and when bound, it prevents
transcription -- that's negative control.  Both positive and negative
control are known to occur in a wide variety of eukaryotes, so in this
sense the lac operon is a fairly good model for understanding gene
regulation in eukaryotes as well. 

There are plenty of differences between gene regulation in eukaryotes and
prokaryotes as well.  Eukaryotic promoters are large and highly variable,
and initiation of transcription in a eukaryote usually involves binding of
a fairly large number of proteins in the promoter region before RNA
polymerase can bind at all.  Still, the basic mechanisms found in the lac
operon make a good starting point for understanding regulation in all
organisms.  

Therefore, I'd say that yes, the lac operon has wide applicability.  It's a
very good model for starting to understand operons generally, which are
common in prokaryotes and found occasionally in eukaryotes.  It's also a
pretty good model for understanding the basic principles of regulation in
eukaryotes as well.