If I'm sitting down for a serious few hrs poker I usually play 6-8 at a time, although maybe one or two more if I'm feeling particularly 'turned on'
I don't really find it particularly tough - at the $10-$30 level, even up to $50, I know I can play my TAG game and do well, so I dont need to get involved in too many hands. The decisions are all automatic, a quick glance at the cards, position and stacks, do the deed, next table.
Like you say, if you get heads up say on a couple of tables while still having 3 or 4 other shorthanded tables on the go then it can get hectic and from time to time i'll miss the odd hand or make an error in a decision, but never particularly important ones, and its quite fun in its own mad way.
To that extent it does remove a lot of the enjoyment from poker - you dont have too much time to sus players out or think too deeply about hands - a definite downside.
Re: 'sacrificing' tournies - can't say i do. I think if that was happening regularly then I'd be playing too many at once. Of course, by chance a few can get shorthanded at the same time, but even then, you can just leave certain hands to time out and fold rather than clicking fold or whatever to ease a bit of pressure.
ROI wise - I think this more boils down to what kind of player you are.
If you like to sit and think about hands, read players, make moves etc etc then your ROI will decrease when multitabling, and although your overall profit per hour or whatever will increase given the increased turnover of STTs, it'll come at the cost of losing a lot of the enjoyment you get from the game, perhaps for not significantly more profit.
If, like me, you tend to make automatic decisions anyway, then your ROI isn't too badly affected. If I'm sitting playing one table, I don't tend to analyse things too deeply, which is probably why my cash game play has never really developed. I've no problem disecting hands when given time afterwards, but in game I like to sit there, have a quick stock take of all the necessary info, know what course of action will make me money in the long run, and get it done. So whether im playing 1 table or 8, I don't think my decisions would be particularly different, therefore ROI isnt too badly affected I dont think.
I think its quite easy(through multitabling practice) to develop the instant decisions necessary to become a multitable robot - whether its a desirable thing is another matter entirely.
Oh, and wouldn't bother with Turbos - all the usual rules about if you are a winning player then you want properly structured tournies to maximise your edge still apply, and you'll get timed out far more often, and these do make a bit of a dent in the ROI in my experience.