I have a whole database designed using identity columns for clustering + pk.
It work pretty fast on insert/seeks but i've seen a few problems:  
1. the index fill option is useless because the inserts happen only to the end of the index  
2. more storage space. I have tables with tens of millions of records and 1 int takes up space by itself. Each table with an identity column for it's pk has to have another index for business seeks, so even more storage required.  
3. scalability. This is the worst problem. Because every insert goes to the end of the index, each insert will stress only the end of the index (allocation, io for writes, etc). By using a business key as a clustering key you can distribute the inserts evenly on the index. That means that you just eliminated a big hotspot. You can easily use more files for an index, each file on a separate drive, each drive working separately.  

I started changing my tables from an identity columns to natural keys (maybe separate for clustering & pk). It just feels better now.  

I would suggest the following:  
1. use as a clustering key the right columns in the right order as to optimize the most frequent queries<br/>
2. use a pk the right columns that make sense for you table