"Is it any good?" I've never owned one, but I'm assuming it's pretty good, if you're looking for a mirrorless camera with interchangeable lenses.
Asking whether pros would buy one and comparing them to a 5D is pretty meaningless, to be honest. They have more differences than commonalities when you get past the fact that they're in the more expensive tier of enthusiast/professional model cameras. You might as well start comparing motorcycles to stock cars. Why do you care which is "better" for "pro work"? How do you define pro work? Are you going to be shooting professionally? Someone who is shooting a wedding probably isn't going to have much use for a compact mirrorless camera with a limited lens selection. But it could be right up the alley of someone who is traveling and wants something with a smaller footprint, without sacrificing the quality of a larger sensor and the versatility of multiple lenses.
Less of a question of "is this good" and whether it's "good for you".
As is usually the case with specialized gear.
Also, there have been firmware releases that helped improve the camera. And the new product announcements from Fuji on the X100S and X20 show that the X-Pro1's successor may pretty much address most of the issues people have with it.
That said, it's a matter of how you use a camera and personal preferences as to which one would be a better choice. The XPro1 is a mirrorless compact, not an SLR. It's a pretty big shift in paradigm/size/weight/controls and system options. Not to mention it's got an ASP-C sensor, not a full-frame. OTOH, a 5D classic (Mark I) is super old. Doesn't even have liveview, let alone video capability, is only 12 megapixels, and isn't any great shakes on the AF, either, but can be found for $500 these days.
Street shooting or photojournalism would put a priority on small, light, unobtrusive gear. A 5D3 is none of those things. During the London riots of 2011, many photographers were targeted by looters, getting mugged and beaten for simply carrying a camera. You really think a 5D3 with L glass would have been an advantage in that situation? Sure, you could get some rocking great low-light shots--until they're stolen from you. Meanwhile, your competitor who used a smartphone or P&S not only got the shot and kept it, but also managed to evade attackers.
Sometimes small is good. Sometimes image quality is good enough. Sometimes it's better. Sometimes ergonomics count more than the number of pixels.
As I said: go read Zack Arias's review. He is a pro. He mentions using the XPro1 for a paid job, and why he did it, even though he also has a PhaseOne medium format digital camera, and a 5DMkII.
I just sold my backup d700 and decided to pick up an xpro1 and primes.
I did get the x100 a year or so ago and loved the idea of the camera and it was a joy to use, it pretty much brought back my love for photography. Seemingly I ended up with bigger and better cameras following my development which ultimately led me to take less pictures because I didnt want to carry such a weighty bag.
Since getting hold of it in DEcember I have added lenses to the kit and updated firmware. Really love the camera, size, weight, functionality but most of all the in camera raw processing. I find it hard to replicate in LR. The quality is amazing.
Most of all it gets me shooting and obviously with primes gets me thinking more about the framing.
Comes highly reccomended from me, x100 also however I found the fixed lens limiting to my style of photography.