I'm not sure what it means for sacraments to be 'valid' or 'invalid'. Sometimes I know that those who are critical of modern innovations in the liturgy are accused of Donatism, but it is important to recall that Donatism involved saying that the personal worthiness or sinfulness of the priest affected whether or not grace was present, even when the forms were those of the Church.
It is quite another matter when the forms themselves are altered -- how much is too much? Luckily for me, the Orthodox don't go in too much for deciding on 'validity' of this or that. We draw the line back quite a bit further. There is a sense in Orthodoxy that while the Body and Blood of Christ may certainly be present, it is entirely possible for them to be "contaminated," if you will. This also goes for us being able to do the contaminating, when we examine ourselves to see if we are truly prepared to received communion. All too often, I am not -- so I don't.
I don't think that amongst Orthodox who travel a lot that it would be particularly uncommon to decide not to commune at that particular parish that they are visiting, if sloppiness in the serving of the Liturgy were to be obvious. It would be rare for someone to leave (although a "Clown Liturgy" would do it. :-) I know you were being tongue in cheek, but it is just too ripe to resist!)
It's not so much a sense of passing judgment on priest or parish as of personal caution in the reception of the Mysteries.
LOL I do tend to get myself in trouble sometimes. Well, I will can the clown liturgy ref in the future as it obscured my major point.