I'm going to break this up into the incumbents and emerging companies.
Hitachi Data Systems
Strengths: Performance, capacity, and the fact that all their software runs on both the mid-tier Thunder line and the high-end Lightning line. The ability to use the same software means that it's easier to manage your storage. The new Thunder 9585V is an excellent value for the performance, capacity and software functionality.
Weaknesses: They undersell their software capabilities and they need more channels to market for the Thunder line.
Strengths: Broadest storage, software and services portfolio. Prepackaging and pre-configuring of storage and software to reduce cost and complexity. CLARiiON CX700 is excellent value for the performance, capacity and software functionality.
Weaknesses: The CLARiiON and DMX line run on different technologies. EMC's software portfolio is enormous and some products overlap, especially the replication technologies -- and that was before the acquisitions.
Strengths: Excellent, modular building-block approach to performance, capacity and connectivity. The acquisition cost of the InServ Storage Server includes basic storage management features through an intuitive GUI. Innovative software Thin Provisioning allows users to allocate as much logical capacity as needed over an application's lifetime. Physical capacity, however, is drawn from the pool only when application writes occur.
Weaknesses: As an emerging company, their biggest challenge is recognition, reputation and market penetration.
Strengths: Modular building-block approach to performance, capacity and connectivity. The Isilon IQ platform was designed specifically for the storage and high-performance requirements of digital content such as audio, video and images. A distributed file system eliminates the capacity limitations inherent in NAS and it allows for automatic capacity balancing.
Weaknesses: As an emerging company their biggest challenge is recognition, reputation and market penetration.