To add to Seamus's answer, siunitx combines the functionality of SIstyle and SIunits, hence having a new name. As a result, both SIstyle and SIunits now have 'bug-fix only' status.
siunitx can operate in two ways. The recommended way is to require that units are used within the arguments to macros \SI and \si:
\SI{10}{\metre}
\si{\kilogram\metre\per\second} % Note no `\usk' here
It is possible to load the package such that 'free-standing' units are defined (\usepackage[free-standing-units]{siunitx}), but this can lead to some macro-naming awkwardness and also makes some functions less reliable.
The package defines a number of units 'out of the box', but these are all SI units. For the next release (v2.3), I will be adding abbreviated units to the set that are defined automatically (currently these are a load-time option):
\SI{13.4}{\kg\per\V}
Binary units are available as a load-time option. (I may make these automatic for v2.3: I have not yet decided.) New units can be defined in the preamble
\DeclareSIUnit\lb{lb}
...
\SI{100}{\lb}
or you can include units without any definition
\SI{60}{mph}
\SI{100}{\SIUnitSymbolDegree F}
\si{psi}
Settings in siunitx are controlled by the \sisetup macro. For the spacing you load with SIunits, the options needed are
\sisetup{number-unit-product = \:, inter-unit-product = \:}
As Ulrike comments, a change to math-celsius seems needed here
\sisetup{math-celsius = {}^{\circ}\kern-\scriptspace C}
I think that looks better over all than the current definition: I'll probably change the standard setting.
Best Answer
You don't have to define it, but I recommend it as it's now a non-standard unit. The reason it's been deprecated is that the BIPM have removed it from the SI Brochure. As a matter of policy, I only predefine 'official' units - there are simply too many other units that people could argue for that I'd have to make a judgement on.