...I think most people use Costar for sales, but maybe most also use it for confirmed leases as well. The leasing data from Costar requires purchase of the full suite of products and is a lot more expensive.
Maybe my observation will be useful. I use CoStar lease and I'm proud of it.
As many of you know, obtaining proprietary lease information from PMs and brokers is more than a little dicey - even when they are forthcoming they're not really forthcoming, you know? There's no credible confirmation in the true sense because I know from actual records that less than half of what I can confirm is the real deal. Lease terms and conditions also vary more widely than purchase/sale agreement terms and conditions, so unless you're looking at the lease - probably confidential by the way - you may not be getting the complete scoop.
I also have a problem because I appraise a lot of shopping centers and larger office buildings here, so have a lot of confidential client information that I can never pass along as a lease or expense comparable. I'll sometimes include a blind "confidential" comparable when it is very relevant, but rarely because I don't want anyone to suspect I'm "making it up" and there's usually enough data anyway.
So, my solution is to use CoStar (and LoopNet when relevant) for evidence that a lease has taken place. I report public information (what the record says), and depending on what I really know about that property, the broker, and the market-standard, I'll adjust the indicated lease rate for a "negotiating margin." I also adjust asking rates the same way.
This is credible in my opinion because asking-rates are really what tenants are shopping against - the total cost of occupancy (base rent plus reimbursements). Tenants aren't making their minds up on proprietary information, either, and brokers are closing the deal based on the tenant's financial ability to pay the asking rate and not "fair market rent."
Sorry, it can't get more empirical than this but it's all I've got. Would this work on the witness stand, the ultimate test? Well, how'd you like me to follow-up on your "verified" rent comps and reveal you were not told the complete truth? I've seen an opponent sweat through a suit jacket over issues like this. I'm more comfortable explaining the truth than hoping not to get caught.
So, CoStar Tenant is useful and I believe it improves the credibility of my rental analysis - is it perfect? Far from it, but knowing that down the street ABC LLC took 4,123 SqFt in #200 where the asking rate was $25/SqFt plus tenant electric is a lot better than simply throwing-up asking-rates or (worse yet) "verified" leases from properties that are not comparable.
As Hal Smith, MAI, CRE etc. used to say at UF, "That's my story and I'm sticking to it."