I haven't, but I believe I may have personal experience that can speak to it anyway. First, I had both hips replaced many years ago and can attest to the effectiveness of joint replacement in terms of increased flexibility and no pain. As well, I know several people who've had hip replacements and all have had excellent results. However, finger joint replacements are not as common and there is been problems with it. You can read about finger joint replacement here (keep in mind that link takes you to a website by the people who make the joint replacements, so it's trying to sell you on it and therefore likely to mention mostly positive aspects of it) and I also found a couple of other sites that are more objective (here and here). If you google 'finger joint replacement' you'll find a lot of websites with more information.

You don't mention which joint in particular will be replaced and which finger, both of which are relevant to your question about joint fusion. I've had RA for a long time and for decades, there was no effective treatment. All of my fingers are fused to various degrees, some completely, some partially and I have limited movement in my knuckle joint. It does impact the way you do things, but one of the defining aspects of us humans is that we adapt can to almost anything. Could you splint the affected finger for a while to mimic a fusion? This might give you a sense of how it would impact you. I think your best bet is to do a lot of reading about the issue, try leaving the question in different forums throughout the Internet (Yahoo groups may have an RA group) and then go back to your doctor and ask a lot of questions.