We had several 60ST30s in house (two "long term" - that is, in excess of 500 hours usage), along with one working 60S30 (& one non-working, which doesn't count...).
Replacement for those is a 60ST50, which is our current "HT" set.
We do NOT: Game; Use our plasmas as Super Larger Computer Monitors; OR View 3D content (have Pan 3D player, Avatar 3D, and the ST50 - but too cheap to spring for glasses, as the 2D version of Avatar looks Outstanding as it is....)
That said, would consider the ST50 a Significant step up from the S30 - but, to be fair, also considered the ST30 a clear upgrade over the S30 (had 1 sample of each of those running side-by-side for a while). (And any of these panels easily beat our two older Pan plasmas for Blu-ray Viewing - although the older, smaller, panels hold the edge for viewing Less-Than-HD material.)
If you placed a GOOD 60ST50 next to a GOOD 60ST30 (no green blobs, pink tint, yellowish cast, vertical "shadows" - or Whatever - on either screen), would expect the ST50 to be judged the clear winner.
HOWEVER - OTHER THAN a Growing Pink Tint - the two 60ST30s we kept for over 500 hours produced a Very Satisfactory PQ, for our usage (see "We do NOT..." list), and if one of those sets was still serving as our primary panel, would not be willing to move to a ST50 - unless, of course, it could be done for Very Little Expense.
In the absence of a Specific Complaint (green blobs, pink tint....), the 60ST30s are fine HDTVs, and most viewers will be extremely pleased in their display - again, barring a side-by-side comparison.
Also, the 2013 panels are now only a "few" months away: as much as the ST50s represented an Improvement over the ST30s, it makes sense to see what 2013 will bring (especially as Further Improvements have already been hinted at).
OTOH, if one has the wherewithal to BUY a New Panel EVERY YEAR: Why Not pump up the economy a bit???