Like Paul said. Spring doesn't "replace" hibernate. Hibernate is a great tool, but Spring + Hibernate can make things a lot easier to do.
As far as disadvantages to Spring. I'm sure there are some somewhere. (Can't think of any right off the top of my head.) But Spring as a tool, isn't really a replacement for everything, but a way to make things easy and simple. Things like the JdbcTemplate, HibernateTemplate, JmsTemplate, DI (IoP), Quartz, Spring Security (Acegi), Spring Web Services...really do make things simpler. A lot of times these things are just wrappers on top of other technologies, but these wrappers make it possible to code with very little setup and boiler plate code. Again, back to the HibernateTemplate, this doens't replace Hibernate, as all the work is done by Hibernate, but by using Spring + Hibernate, you can write DAO's and functional code very quickly.
Try working with it for a bit. Do a couple of tutorials that can be found online. (Google Spring tutorial) Check out the example projects that are bundled with the Jars and Source. There are quite a few different ways to do things, and the Spring team does a good job of including example projects.
Nick