For a simple answer...
You are correct...CEREC is very cool and promising.
That being said, we cannot expect restorations to last a lifetime as
this has not been demonstrated, but even our current materials rarely
last a lifetime.  Yet, it would not be unreasonable to expect an
exceptional 5-year survival rate.
For small fillings (less than 1.5mm in diameter), I don't see any
benefit to CEREC.

More details...

CEREC is the name of milling unit/process produced by SIRONA, a
company that sells a CAD/CAM milling machines to dentists.  A CEREC
unit will machine a block into the shape of a restoration for the
dentist to place intra-orally.

Stating that you are getting a CEREC filling doesn't communicate what
restorative material is being used; the strength and durability of the
restoration is related to the material used.  Typical CEREC material
are resin composite or one of various ceramics (Alumina, Zirconia,
Leucite, Feldspathic) which can be purchased from various companies.

CAD/CAM is beginning to become very common in dentistry.  Accuracy of
fit has been demonstrated as low as 15-20µm which is below the film
thickness of cement and the strength of the new Zirconia YZ cores is
approximately 900MPa.

Yet these materials and methods are new and do not have the proven
history of metal-ceramic porcelain which was first patented in 1962.
The longest reports published regarding CAD/CAM restorations are
limited to 5 years, if I am not mistaken.  This shouldn't detract from
the promise of the new materials, only put perspective on the scope of
history available to other methods.  It is uncertain if the brittle
nature of even the strongest ceramic will accrue sufficient damage to
result in catastrophic failure.

I have used CEREC 2 and the recent CEREC 3D in addition to other
milling units available.  The technology has come a long way, but it
is still susceptible to user error.  The technique for taking photo
impressions is more demanding than conventional visco-elastic
impressions and highly prone to error.  Despite the excellent accuracy
of the machine, I have seen many poorly adapted restorations.

When CEREC is used for same-day restorations, it doesn't allow the
same customization that veneered ceramic produces and I am not fond of
the simple appearance produced.

In regards to size...the blocks are relatively large when compared to
minimally invasive restorations.  There is little reason to use it for
restorations under 1.5mm in diameter; direct fillings are best suited
for this purpose.