If you're going to stay in Paris for a week or more, consider renting an apartment. Particularly if you're travelling in a group that would benefit from 2-3 bedrooms. Apartments are generally cheaper than hotels and integrate you more into the day to day life of the city. They're also a bit awkward to check in and out of and don't come with daily maid service or a concierge, but for a longer stay that's not such a big deal to me.
If you want to stay in the middle of Paris, I can without hesitation recommend Guest Apartment Services. They exclusively manage about 20-30 apartments, mostly on the Ile St. Louis (a very desirable location). The apartment we rented in the Marais on Rue Charles V was fantastic, very well furnished and maintained. I've seen three of their other apartments and all were nice places. Prices for a 2 bedroom range from 330€/night to 690€/night and quality does vary. We got a discount for a month long stay in off-season; I think most of their business is shorter term.
There are lots of other options for apartments in Paris. Mostly you end up renting directly from the owner, which can mean random quality. I liked having a professional service managing the apartment instead of dealing directly with the owner.
They're quite responsive by email. Mention my name if you contact them!