It's a good idea to have a boost gauge, and the OEM unit is ok, but
it's overpriced for what it is. There are plenty of aftermarket
alternatives for the same or less money that you may find nicer. Note
that the OEM gauge is calibrated in metric units (mPa) which is not
necessarily a bad thing, but some people prefer an SAE gauge
calibrated in PSI/in-hg.
One option is the Omori 45mm gauge, which is almost the same size as
the Subaru/Lamco gauge (42mm) and can be installed in the OEM steering
column pod (it's a bit tight) or an Omori pod. Reads in PSI and has
better illumination. I have the OEM three-gauge pod which includes the
Lamco boost gauge, I recently replaced it with the Omori "full
illumination" mechanical boost gauge and I like it a lot better.