I really enjoy Disneyland Paris, but my feeling whenever this question comes up is that if you travelled to Paris specifically to visit DLP you probably would be disappointed. If you're a Disney park fan and happen to be in Paris, however, it is definitely worth a trip.
Disneyland Parc itself is a beautiful piece of theme park design and is getting its act back together when it comes to maintenance. On that front, it's honestly not any worse than MK and probably in fact better. The attractions are certainly plussed versions of what you get at MK without the insane queues. Compared to Disneyland, I'm not sure that's the case. What everyone says about WDS is true. It's also true, though, that you'll have a good time there on attractions like Crush, ToT, and Ratatouille before hopping back over to Disneyland. I've also stayed at Santa Fe, Sequoia (my favourite), and Newport Beach Club, and liked the little French Disney bubble... in part for the weirdness that such a bubble exists in France!
I find it hard to imagine traveling from the US to Paris just to see Disneyland Paris. On the other hand, if someone was looking for an alternative to their usual Disney theme park vacation, you could do a lot worse than both seeing both Paris and spending a few nights at DLP!
Good summary. We visited DLP last year as my wife wanted to get the Castle-to-Chateau medal from runDisney. We'd have not done DLP otherwise, but I do want to go back after all the announced projects are complete. What we saw of Paris was good (only spent one day in the city), but needing to have your head on a swivel the entire time, and having encountered at least one pickpocket before we even made it to the city (and then saw many others throughout the day), was not enjoyable at all. The Tokyo parks are at the top of my bucket list.
The only thing about DLP we didn't care for was the price of the food. And that we missed Phantom Manor. I've been to 3 of the castle parks and still have only seen the Orlando version of HM. BTM at Disneyland Parc is the best version of the ride, hands down.