In the last year we've tiled our foyer and kitchen. Foyer was a bitch - had to remove old hardwood flooring (couldn't be saved) and even after renting this scraper machine with a huge blade, we ended up on the floor with hammers and chisels to get a lot of it up. As a bonus, the hardwood had been laid so soon after the slab was poured that in places, small concrete chunks came up with the adhesive.
As Madman described, we scraped the adhesive as much as possible and used self-leveler in a few spots where needed. Once that dried, we laid the tile and it worked out fine. The subfloor doesn't need to be perfect; you'll see this when you apply the thinset mortar, which is thick enough to equalize minor inconsistencies.
The kitchen, though as much larger area, was ridiculously easy in comparison because they'd just laid linoleum over the slab with minimal adhesive here and there. All we had to do was pull it up and scrape a spot or two. We also used 16x16 tiles, which covered that space in a hurry.
Grouting is what blows, though if it doesn't come out the way you want it, use
this grout colorant . It's easy to use, seals the grout, and is available in a range of colors. Once we decided on the color, it was simple to apply and looks great.