Does anyone use a web based meeting scheduling tool they would recommend?

I have to schedule a lot of meetings with (busy) people outside of my organisation i.e. I can't see their availability in Outlook.

I used to use Doodle, but the free version seems to have gone down hill (only 1 time slot per day), their website is sluggish/unresponsive, and I can't successfully register for a 'premium' trial.

I am looking for a service that will read my calendar and generate meeting slots for invitees to select yes or no for. 

A service that allows invitee's to share their calendars and suggests meeting slots based on when everyone is available  that would be a bonus.

Happy to pay for a decent service.