Hello Redout.
Yes in essence it can be thought of as a sort of Glideslope. It is often reffered to as "the doughnut" because it is a small magenta circle that slides down the side of the PFD Altitude tape.
Whilst it is useful for watching a managed profile there are some caveats - remember the FMGC calculated information is only as good as the information put into the FMGC - keep it up to date!
For example, lets say you have an intermediate constraint at a point in descent i.e FL200 at VOR "ABC". So you leave FL350 in "managed" descent and you're "on profile" so the doughnut is centred. You achieve the required constraint of FL200 and now the doughnut starts to indicate you are going "high" (moves down the PFD Tape) backed up by the PROG Page deviation. The FMGC Software does this irrespective of how many track miles you have left - it's actually not that clever. So you maintain FL200 and your deviation now indicates you are 3000 feet high on the "FMGC Profile" (whereas in reality you may be 8000 feet low based on the 3 X table rule). So if ATC now clear you to FL 180 and you choose Managed descent the aircraft will set Idle and "dive" to try and get back on its (incorrect) profile.
There is a cure for this. If you re-enter any constraint further down the plan (perhaps 220 knots approaching the hold), this will force the FMGC to "look ahead" and recalculate the profile. So when you reach your first level off i.e. FL200 and the doughnut starts to slide to the bottom of the PFD - just re-enter ANY constraint downroute and hey presto, a recalculated profile of perhaps -9999 feet. Now when ATC clear you to FL180 the FMGC says - ok I'm below profile, so I only need to go down at 1000 FPM....not 3000 FPM!
A "DIR TO" has the same effect (recalculte) but if you're in HDG you MUST pull HDG again (so as not to enter NAV) so I always teach guys to re-enter a constraint - much safer and covers all cases.
Hope it helps.
A4