LED's require a fair amount of research. There are simply too many emitters and optics options to just 'go out and buy some'. No single LED is going to match an HID, however with the right components, a group of them can match HID's.
Here are some pro's and con's, though by no means an exhaustive list
Pro's: PAR for PAR, LED's will give you more per watt (2nd place is MH), and produce A LOT less heat (IE they are way closer to the plants, part of the reason they can compete on PAR with MH). As a collection of many point sources you can basically wrap your plants in them. They usually last 50,000 hours, so maintenance costs are basically nil in that time, but when they die you have to buy new emitters. You have a lot more options on spectrum, since you area creating a mix, than you get with HID bulbs. Many of the advantages to LED's are the same as CFL's, however they overcome a lot of the disadvantages to using CFL's.
Con's: They are expensive. They are more expensive than you expect. Most people who don't have luck with them either just didn't buy enough, or didn't buy high enough quality made for the application they were using it for (horticulture), usually it just came down to them thinking they could spend roughly what an HID setup would cost, not realizing that LED IS EXPENSIVE. Unless you build them yourself, you usually have no idea of the real specifics of the emitters and optics, or even the spectrum, and finding par ratings on them will be difficult. When building them yourself you might find it's difficult to find specific information on what emitters or optics to get, as well as how to properly mix for a desired spectrum.
I haven't seen PAR comparisons for anything deeper than 36" in water (almost all my LED experience if from coral propagation), so it's quite possible that current LED's simply aren't going to be able to match the 1000w HID's. Most of the systems I have seen tested were compared to 400w MH systems with high quality reflectors and overpowered icecap ballasts. The LED's were all custom built by the tester, and unfortunately I don't know what emitters, optics, or drivers he used. All I remember specifically was that he mounted it all on PC heat-sinks with small processor fans mounted. The temp on the underside of the heat-sinks stayed within 1 degree of the ambient room temp.
Edit: Something interesting relating to the future of LED, companies are starting to release commercial plasma based lights. These things are quite impressive, and are as close as you can get to having a little piece of sun of your own. Spectrum and intensity can be fine tuned at the turn of a knob. From what I understand they are incredibly energy efficient as well. The only real downside is that the ones commercially available right now produce a lot of heat. I know a couple of coral propagators who got a few before Luxim (
http://www.luxim.com) released them, and their feedback was incredibly positive.