I can definitely understand why sitting up close during games would be rather enjoyable. I have, after all, splurged a couple times to get seats a couple rows back from the field at Nationals game. And when I go to minor league games I almost always get the closest seats possible.
To be honest, however, I only sit up close at minor league games because they're usually cheap (about the same cost as going to a movie). When it comes to MLB games, however, I simply can't afford to pay for super close seats. The next best seats, in my opinion, are in the infield upper deck.
There are various reasons why I think these are the next best option. For those who are interested in the best viewing opportunity, there's the plain and simple fact that they're the INFIELD upper deck seats helps to show how you're closer (at least horizontally) to most of the action than the seats helps to explain how you're closer seats than somewhere down the baseline or in the outfield on the first level. By sitting up high, you also get a good perspective on how plays develop and a sense of how the defense changes its positioning depending on the batter and game situation.
In ballparks that don't sellout, there's also usually a fair number of empty seats in these sections. As a result, you have more room to stretch out. When you consider that a game usually lasts three hours or so, having this extra space can be nice -- especially if you're carrying a few extra pounds like me.
When you combine the fact that the view is better in many aspects than those in the outfield and down the baselines with the fact that these seats are also cheaper (at least at Nationals Park), I definitely think these are the seats that offer the most bang for the buck.