I've had a radioRA system hooked up to my elk for about 3 years now.. It's been rock solid since day 1, never missed a command. The switches have a very nice solid feel to them and they've put up with a lot of abuse, and most of them are all used (bought secondhand off ebay). As for the interface the only one that i'm aware that works with the elk is the radioRA rs-232 interface, which you set a few jumpers in a M1XSP, and set up your lights as zones or phantom buttons in the RS-232 and you're good to go. I know a lot of people have commented on the price of radioRA as being a sticking point, but i waited it out on ebay and searched for deals and got most of my switches for $20 or less.. now that radioRA2 is out, it's driving down the prices on the pieces even further, i even picked up a 2 spare repeaters for $10 each. It's been as rock solid as a hardwired system without any of the prewiring.. if your boxes have LNG, you're good, some switches don't even need the neutral. Like all system's (other than hardwired) RA can't be said to be 100% failsafe, but the frequency it operates on has never had any interference with me ( a lot of people complain UPB doesn't work for them due to 'dirty' power lines) but most people who have had complaints have solved it with the addition of an extra repeater to cover the area.. the only other caveat with RA and RA2 is the system size, limited to 32 switches IIRC without a chronos bridge and 64 with a bridge. It's never been a problem for me, i'm maxed out with all the ones i want to control at 28 switches.
As for a better system, i hear RadioRA2 is even more rock solid than RA1 with more 'built in' features, but the elk handles all the extra things i need the lights to do.. the new RA2 system has the serial interface built in to the main repeater so less pieces to buy, but the pieces are more expensive right now. There was someone on cocoontech who was a retailer for RA2 i believe so you can speak with him if that's the direction you want to go in. Also i don't know if elk supports RA2 yet, but it probably will soon as HAI does.. in the end, pick a system the elk supports natively through a M1XSP, it'll make integration a lot easier.