It's a personal thing, but I just don't get flavored coffee. If you want a milkshake (because that's what most flavored coffee ends up tasting like or turning into), then just get a milkshake. Don't ruin a perfectly good cup of coffee by filling it with ghastly concotions dreamed up in some New Jersey foodstuffs lab.
The argument against flavored coffee is simple: Regular coffee tastes good. If you want it sweet, you can put some sugar or sweetener in it. If you'd like it a little thicker, you can put some milk in it. You can have it strong - espresso has all the wallop of two cups of coffee in about two ounces. You can have it creamy, like a cappuccino. You can mix it with brandy or irish whiskey or bailey's or tequilia and cinnamon.
Just don't mix it with vanilla soybean non dairy creamer that you got from the sugary treats aisle at the local supermarket. It doesn't make sense to ruin good coffee with that kind of crap when you have so many other better choice to make when it comes to your daily cup of joe.
That said, there are some types of flavored coffee that are actually good ideas. These are the ones that actually roast the coffee beans with additional flavors added in, instead of dumping overly sweetened creamers into an already made cup of coffee.
Take, for example, almond flavored cofffe. You'll find places that start with Arabica bean coffee, then add roasted almonds to the blend. Almonds are among the sweetest of nuts, and have a delicious, butter flavor. If you find arabica bean coffee that's been roasted with almond flavors, then you've found a flavored coffee that's worth sampling.
Incidently, almonds aren't actually nuts. They're seeds that you'll find insde the fruit of, yes, the almond tree. They're actually closely related to peaches and nectarines genetically. Though i'm not going to recommend peach or nectarine flavored coffee!
Antoher flavored coffee that makes some sense is amaretto flavored coffee. Amaretto is a popular liquor from the north of Italy, the Lombardy region, and it is from this tradition that amaretto flavored coffee comes. I personally find it to be too sweet and syrupy, but I'm not a fan of liquors in general and tend to prefer hard liquor like whiskey, vodka or tequila if I'm spicing up my coffee in that way.