What do you like best?
Deloitte has a ton of resources that can help get large projects done in short order. And while they can be a bit pricey, they also have consultants with various levels of expertise and offshore resources whose rates can be blended together to help keep costs down a little bit.
What do you dislike?
Just like any large consulting company (or any large company in general), there are always going to be good consultants who really know what they're talking about and can make excellent suggestions for how to solve problems and bad consultants who aren't actually as knowledgeable as they seem. The hardest part sometimes can be determining which consultants fall into which categories and then following up with the company to replace the people you aren't happy with.
Recommendations to others considering the product
This is not specific to Deloitte as much as it's advice when considering any large consulting company in general - make sure you have well defined requirements and subject matter experts in house who can help drive whatever projects the consultants are working on. When used purely for staff augmentation, an organization like this can do some great things, but if you let any consulting firm completely run your projects from start to finish with minimal involvement on your end, you're asking for trouble.