Oracle and IBM databases: Disk-based vs In-memory

The case for in-memory databases (IMDB) can be made in three simple points (1) performance - data is kept in RAM so no disk I/O limitations (2) HA with built in fail-over (3) support for relational schema and SQL. Current disk based RDBMS can run out of steam when processing large data. Can these problems be solved by migrating from a disk based RDBMS to an IMDB? Any limitations? To find out, I tested one of each from the two leading vendors who together hold 70% of the market share - Oracle's 11g and TimesTen 11g, and IBM's DB2 v9.5 and solidDB 6.3.