So, if I understand it correctly, the expansion is not a separate campaign, but adds new areas to the existing one, much like Tales of the Sword Coast for Baldur's Gate?

Is it worth playing the game without the expansion first, or do I just start with it installed?

Definitely play with the expansion. It's the best content in the game.

As noted above, play with the expansions. Not only they add a lot of good content, but they also raise the level cap by 2. Without them you can reach maximum level long before the end of the game. There's no reason to play without the expansions, if you have them.

Aside from the raised level cap, does the expansion affect areas of the main game? The new monsters/items/enemies included, are they added to original areas too or just the expansion?

It doesn't affect the areas of the base game with the exception of your Keep (which adds expansion related dialogues) and dialogues for new companions.

The content is great, but to me it's just too much of a digression. You are rushing to stop the Leaden Key in the main game, and suddenly you're off to far away land in a completely different quest. The integration is practically non-existent, they should have formally made it a separate campaign.

That being said, be sure to enter White March part 2 with a really strong party and also save often, because White March pt 2 can end in game over and prevent you from completing the main game altogether.

Heh, this is so true for many expansions to RPGs. Saving Icewind Dale from the greatest evil ever? Well, that can be put on hold while you go on the expansion quest.

Thanks to everyone for your answers.

As was said, you should certainly start with it, there is no reason not to. It adds some very good stuff and it doesn't directly influence or change the vanilla game. There is the drawaback, though, that it is hard to tell when to visit expansion stuff and there is not really great way to do that to not either make the endgame or the expansions a bit too easy.

Ah, so if you visit it late, it is a bit too easy. If you visit and finish earlier, the original end-game becomes too easy? Is that it?

Pretty much. I can't remember if PoE already had the "level adjust enemies" option. I think not. But I kind of like feeling powerful after all that fighting to get there, so I leave this off anyway.

If your characters are more experienced there's a notification which allow you to increase the level of monsters after you enter a certain area in the base game. I don't remember whether there's a similar notification in the DLCs.

Exactly. The game offers you the option to increase the difficulty of the game if you are considered too high level but I found it is not enough and from what I have read many people concur.
But I understand it's hard to do. I am not sure I have played RPG which had expansion accessible during the main story and not making any part of it unbalanced.
It is price one has to pay for it but I personally think it is worth it.
If you are going to stick with it I think best time go play expansion 1 before visiting Twin Elms (it is quite obvious in main story) and I am still not sure where to do part II. I once did it right afetr 1 and it was quite hard and then rest of game was just walk in the park and second time I did it just before the very end of the main story and it was a bit on easy side but overall better I think. Well, the part II presents maybe the bigger threat to game world than main story so it quite justifiable to make a detour to take care of it.

I agree.

OK, I'll go with this and see how it works out.

Right, now I remember! There was a question if I wanted to make the game harder. I PoE2 there is a global level adjustment option for enemies.

If you are level 9+ when entering The White March, you will receive a message asking if you want to level scale. This will boost the stats of every NPC, including in the base game. The drawback is that it makes the endgame content unfair, since enemies will have much higher stats than your party after hitting the level cap.

Were you on PotD?. Llengrath becomes almost impossible with level scaling turned on.