The most important difference in terms of
usability is that 'droch' and 'deagh' are ONLY used directly before the noun (and lenite it where possible).
'S e droch naidheachd a th' ann.
It's bad news.
(or, less common, 's e naidheachd dhona a th' ann.)
BUT, if you really want to say
'the news is bad':
Tha an naidheachd dona.
The same system applies to 'deagh' and 'math'.
There are subtle differences in meaning and register too, but the ability of 'droch' and 'deagh' to precede the noun lends them emphasis, so they are often used in emphatic situations, i.e. particularly bad or good.
The frontal positions also means they lend themselves to compound nouns and adjectives, e.g.
droch-nàdarrach: bad-natured
deagh-aithnichte: well-known
And both 'droch' and 'deagh' are used a lot in spoken/colloquial Gaelic.
Think that's enough to be going on with!
Consult Colin Mark's dictionary for idiomatic usage.
now all I need is someone to explain the 'cofaidh' thing... 