I dunno, lances are very powerful, and generally if you know the trick it's pretty easy to do some serious damage with them. With that said, they're basically a one trick pony (smash), and if that doesn't work you're 100% out of luck. When it does work on something, which is a lot of stuff, it can wreck stuff.
Next time I get on (in a few days when ole hillwen comes out) I'll give you a bit of a rundown. For now, all I'll say is they excel at chaining smashes, and the extra range on the lance makes all the difference in that. You can get a smash off after a lance charge, you can get a smash off after an assault slash (if the slash knocks them back), generally if it knocks them over/back you can use it to get a smash. Honestly that's about it. It's easy to chain together charges and slashes and slip a ton of smashes into the string while basically preventing the enemy from doing anything.
If you've gotten to the point where you need to counter/windmill enemies away, you generally want to switch to another melee weapon, because once the enemy has that much pressure on you and is in that close, the lance doesn't really have anything it can do, besides hope to land a smash after a mill or counter, which is very dangerous if the enemy manages to start moving before you reach them.
Enclosed areas are a bad place to be as a lancer, naturally meaning dungeon rooms and some shadow missions are harrowing experiences, and you should basically forget about fighting in a narrow hallway if you can't kill it in one or two shots. With that said against things like dragons they are fantastic, and even in dungeon and shadow mission rooms are servicible, there's just a bit less margin for error. Heavy stander can easily ruin your day, as it will prevent lance charge from KBing I think, which can put a violent end to your smash streak.
Not a main set by any means, but very powerful once you get the hang of it. Is it worth sacrificing a second slot for another melee weapon to cover you if you mess up? Depends on what you're going for! It could certainly be a detriment for the more multi setted folks in the game. And indeed most folks would prefer to carry around a wand/bow. But if you're going strictly melee and care for nothing else, you've got your second slot right there.