I have only attended the traditional Latin Mass my entire life, Pope Pius XII version.  Some recent casual buzz in certain circles in the parish I go to about the John XXXIII Mass has provoked a personal interest in knowing more about it. How does it differ from what I attend now? Is it Valid? Invalid? Etc.

A list of changes in the Mass by Roncalli can be found at:

http://www.novusordowatch.org/john_xxiii_mass_changes.htm

I'm not sure why any of these changes would invalidate the Mass.  The invalidating feature of many Masses using the 1962 Missal is the fact that priests ordained by Conciliar bishops are not Catholic priests--they remain laymen--because the changes in the Rite of Orders for the consecration of bishops is invalid.

Many priests of the SSPX initially used the Mass of Pope Pius XII.  Archbishop Lefebvre allowed priests to use that Missal or the 1962 Missal early on.  The archbishop later changed policy in the SSPX to be conciliatory toward Rome and required the use of the 1962 Missal.  A Mass said by an SSPX priest who was ordained by an SSPX bishop would be valid.

The complaint of many people is that Roncalli had no authority, being a non-Catholic anti-pope, to make the changes in 1962 so the use of that Missal is illicit.  Another complaint is that the changes made by Roncalli was a precursor to the Novus Ordo.  The second complaint is often levied against the Missal of Pope Pius XII as well though I do not subscribe to that opinion.