Jay Thompson pondered a while back whether or not to require registration from IDX users on his web site.
This is potentially an even bigger issue to ponder - whether to have a link at all (to IDX or not to IDX, if you will). But you say, "How can that possibly be bad for lead generation, isn't that what helps drive people to my site?" Perhaps, but maybe, just maybe, having the link gives them all they really wanted to begin with (including the chance to avoid dealing with a live REALTOR) and they no longer need to contact you (or have you contact them) to give them that same information, or some subset of it. If you don't believe me that this might be real, read this article from Lawrence Schoeffler in a recent Realty Times edition.
I will note that the article seems a little ambiguous as to whether or not registration-required IDX links have the same negative lead generation effect as non-registration-requiring IDX links. One thing is very clear - a web site with something of value to consumers, who have to register to get it, generates leads over and over again(something "Mr. Internet", Michael Russer preaches very hard).
Some people reading this might say, sure, but what good is a lead if all they want to do is find a little information now and they really aren't buyers or sellers (at least not in the immediate future). I think the answer is obvious - you have to start somewhere to build a relationship and by the time they are ready to actually hire someone, let's face it, a lot of the time the deal is already awarded to someone in advance. Even if they do interview more than one agent/team/firm, the consumer has quite often already made up their mind, or nearly so, and therefore the interviews are only to get extra information from others and to solidify the decision they have mentally already made. Agents criticize lead generation sites (in part because of the high cost of their leads) because the leads are often not yet doing anything. Welcome to "isn't that the same as passing out your card at a PTA meeting"? Leads take time to turn into cash. In the corporate world I have worked on a deal for as much as three years before it became "real" and then commissionable (generally those are well 
worth it, though, when they happen!).
For our residential site, we had all the forms filled out for an IDX link for our site, even sent them into the MLS. A week later we had second thoughts and they hadn't yet charged my Visa, so we called them to cancel and (by the grace of God) they had only started to process the request that morning, a week after getting all the necessary paperwork. To me the message from on high was clear, at least for Angelic-RE.com - no IDX! Let your server relax and skip it.
Posted by 
;)
;)
Gab,
I'm using a VOW which does require a sign-in form. And, yes, there's a lot of carrots. And some deer too.
Deer? Yup, those are "leads" who fake name and phone...and wanta come into the patch and graze.
I'm strongly thinking about the Steve Davis technique...that if I can't contact them...then they're deleted from the system. Again, the Mike Ferry adage: You can't loose what you don't have.
And, I suspect that I'll only have a few deer. Mostly rabbits.