CE560 Captain reports descending early on the ELDEE 5 arrival to DCA. VNAV is not selected after 8;000 FT is set in the MCP altitude window.

Narrative

We were level at 15;000 FT; proceeding to DOCCS on the ELDEE 5. ATC cleared us to descend via the ELDEE 5. The CE560 FMS system is the Universal UNS-1 series; with software SCN 80X. This system can sometimes be problematic in a SID/STAR procedure and requires more attention from the pilots than a Pegasus or Proline FMS. Point being; there are more opportunities to make mistakes; as the interface of the FMS is not as intuitive or elegant as some of the newer systems. The inherent design also makes it more difficult for the pilots to crosscheck each other; with regards to what the aircraft is programmed to do next. After clearance to descend; the pilot not flying dialed 8;000 FT into the altitude select window and assumed the pilot flying would engage VNAV in order to adhere to the flight profile. Unfortunately; VNAV was never engaged and the aircraft reverted to a pitch mode that allowed it to descend below 15;000 before the procedure called for it. ATC alerted us as we simultaneously noticed the uncommanded descent. We immediately stopped the descent and climbed back to 15;000. ATC did not tell us there was a problem from their side; so no phone call was made with Potomac post-flight. Having flown the ELDEE hundreds of times in various aircraft (MD-80/737/757/CE560); it still remains challenging to keep up with the continuous reconciling between the STAR and the FMS. This procedure is one that requires aircrew to double and triple check the aircraft's progress against the charts; and right when you feel comfortable; complacency rears its head! The CE560 makes the procedure even more challenging; and even with 7;000+ hours of experience; I have found the possibility of mistakes on the ELDEE as a clear and present danger.