I have some experience with MS-SQL 2005 (publisher) and SQLEXPRESS (subscribers) with overseas merge replication. Here are my comments:
1 - Is replication built in, or an add-on/plugin?
Built in
2 - How does the replication work
(high-level)?
Different ways to replicate, from snapshot (giving static data at the subscriber level) to transactional replication (each INSERT/DELETE/UPDATE instruction is executed on all servers). Merge replication replicate only final changes (successives UPDATES on the same record will be made at once during replication).
3 - Is it easy to check consistency between master and slaves?
Something I have never done ...
4 - How easy is it to get a failed replica back in sync with the master?
The basic resynch process is just a double-click one .... But if you have 4Go of data to reinitialize over a 64 Kb connection, it will be a long process unless you customize it.
5 - Performance?
Well ... You will of course have a bottleneck somewhere, being your connection performance, volume of data, or finally your server performance. In my configuration, users only write to subscribers, which all replicate with the main database = publisher. This server is then never sollicited by final users, and its CPU is strictly dedicated to data replication (to multiple servers) and backup. Subscribers are dedicated to clients and one replication (to publisher), which gives a very interesting result in terms of data availability for final users. Replications between publisher and subscribers can be launched together.
6 - Any other interesting features...
It is possible, with some anticipation, to keep on developping the database without even stopping the replication process....tables (in an indirect way), fields and rules can be added and replicated to your subscribers.
Configurations with a main publisher and multiple suscribers can be VERY cheap (when compared to some others...), as you can use the free SQLEXPRESS on the suscriber's side, even when running merge or transactional replications