Excel is a great number-cruncher. Access is much better at organising and correlating complex data sets, and managing multiple users. You can also set up sophisticated data validation in Access, and handle much more data than Excel can. The total file size limit for Access is 2 Gb, but if you start approaching that it's time for something more heavy-duty.
I use them both. The trick is to know which is best suited for any particular job. Together they are a great combination.