I've never heard the HD500 Fusions, but I've never heard anything good about them either. I own a pair of HD545s and have used them for almost 3 years now and I would highly recommend them for a few reasons. One is that they're supremely comfortable. They have soft velour earpads that go completely around your ears, and a soft padded headband. Mine clamp less than HD600s, which makes them even easier to wear over a long period of time. These are full-size headphones, which is good if you're planning on using them for long periods of time while relatively stationary (at home or the office).
Another thing I like about them is the sound. To me, they sound like kind of a mini HD600. The character of the sound is essentially the same, though the HD600s manage to be more detailed and effortless sounding. The HD545s have somewhat weak low-bass response so be aware of that if you plan to listen to pipe organ music or anything like that (not a lack of bass, just that the response falls off below a certain frequency). Otherwise, their sound is very smooth, light, and airy. They're not the most detailed or spectacular sounding headphones out there but they are very inoffensive and will make pretty much anything sound good without being harsh, shrill, or grating.
The last thing which is a potential advantage of the HD545s is that they're fairly efficient and can be used without an amp. The sound fills out a bit with an amp but you should be able to drive them perfectly well straight from a sound card or CD player. If you found a pair of HD545s for ~$70, they're probably close to the best you can do for that price if you want laid back, open phones and they're definitely the best you can do for that price within the currently offered Sennheiser line, though I can't speak on the new models they've just introduced.
I only have two complaints about my HD545s, and they're relatively minor. One is that the drivers aren't matched as closely as they could be. The right driver is somewhat stronger than the left so sound appears to be a little right of center. This is probably just a characteristic of my pair and not ALL HD545s though you do get better driver matching with HD580 or 600. The second thing is that I have intermittent contact issues with the cords on both sides, though the left one is better than the right. The connection is a little loose and can cut out if you accidentally pull or bump the cable. It can be annoying but it's pretty minor and doesn't happen often. The HD545s originally retailed for over $200, so if you can find them now for $70 or less, they're a steal (I paid almost $90 for mine 3 years ago).
