I am currently using one in my E-1 and have been for 3 months and over 1000 photos. So far, I must say I love it. I got mine (Hitachi 4gb - same as IBM I believe) for $165 on Amazon.com, shipped.
If you don't want to read through all of this here is my summary:
Pro's: Great Value, Huge amount of Storage available, Faster than standard solid state media.
Con's: Admittedly not as tough (although it hasn't failed me), not as fast as Scandisk Ultra line
Would I buy it again over solid state media - Definitely
I do understand that it is not as "tough" as solid state media but at the time a fast card was running about $400 for the same GB and a slow 4gb was still in the $300 range. I was getting by with 2 512's and really needed more storage as I moved in to shooting more RAW. With the 512's I was getting 47 RAW images per card. With the 4 gb card I get 379 RAW images.
Having that many images available to me on one card, I have been treating it as "embedded" memory. The microdrive has only been out of the E-1 twice since I bought it. This keeps it safe from dropping and mishandling. I hook the camera up to the computer when I want to transfer photos.
I think the microdrive technology that has matured over many generations and I am fairly certain it will give me years of great service with no problem. I haven't really noticed much of a difference in power drain but I have noticed that while it is faster than my "slow" Compact Flash memory, it is not as fast as my Scandisk Ultra II cards. In fact it is about 30% slower (yes, I am anal and I timed it). This hasn't been a problem for me since the E-1 has a 12 shot buffer and that is usually good enough for me. But if you shoot in a lot of situations where you would shoot 12 and want to shoot another 12 as soon as possible then may be you need to look elsewhere.
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Tarek