In the old book, pathfinders were usually overlooked by people as they preferred to take Piranhas to use for blocking or AV14 hunting. I'll cover the Piranhas later but I expect we'll see a lot more Pathfinders now. The issue people had with Pathfinders before was that you were forced to take a Devilfish. With people not really taking Fire Warriors they didn't want to fork out for a transport they weren't going to use.
Pathfinders are 11pts a piece with a minimum squad size of 4 up to a maximum of 10. They're cheaper than before but now only have 5+ saves so will need to hug cover. They've actually got the most options of any unit except crisis suits. First thing to note is the array of unique drones they can field. The Recon Drone is by far the most striking as it's huge! Rules-wise it has a place with a Homing Beacon and Positional Relay to help Reserves arrive accurately. It's also got a burst cannon which is pretty weighty fire for a drone. The Reserves shenanigans don't really work all that well unless you're mounting it in a Devilfish though. As ever the Devilfish is a dubious choice for Pathfinders as they need to stay still to use their markerlights. Without it though the drone won't be in a suitable position in time for turn 2 or 3 when reserves are most likely to arrive.
The other drones are the grav-inhibitor drone and pulse accelerator (which has a typo in my version making it BS3 for that weapon it doesn't have). The latter extends their range by 6" but since most of the time you'd be firing their markerlights it feels like 15pts wasted. The former, however, has a use. It's still a bit pricey but it allows you to reduce an enemy charge by D3". Doesn't sound much but I'm sure everyone has only managed a charge by the skin of their teeth. Being able to prevent a charge is great as your opponent is either going to take a pile of Overwatch fire and maybe not get into combat or else is not going to risk it. Either way is fine for Tau players. If you can spare the points I'd recommend squeezing these in.
Pathfinders can also take up to 3 ion or rail rifles. It's difficult to persuade myself to take these at the expense of a markerlight and I don't think they're all that good for the points in comparison. EMP grenades suffer the same problems as they do with Fire Warriors in that they're great for that one time you get charged by walkers or a vehicle comes to close. The rest of the time you'll be wishing you'd spent 20pts on something else.
Taking a shas'ui is probably a good choice as these units are unlikely to be in range of an Ethereal. He can take a Blacksun Filter, for some reason which escapes me, as his markerlight doesn't allow saves and his carbine is short range. If you've got the points you can boost your marker total with drones for him but for 12 pts you get a BS2 marker instead of the BS3 ones for 11pts. Once again Bonding Knives are available but once again I don't think they're worth it to possibly save a couple of models who could only Snap Shot their markers if they regrouped anyway.
Essentially I think the best build here is either two large squads of 10 with grav drones or else three squads of 7 without the drones for the same price. The advantage of three smaller squads is that you can target more units but a lot of the time you'll be wanting to focus fire anyway.
Finally I obviously need to talk about how markerlights work in the new codex. They aren't hugely different from their previous incarnation. They're still a Heavy weapon meaning you need to find a good spot for your pathfinders as they'll struggle to relocate. Speaking of which bear in mind the Scouts rule will allow Pathfinders to redeploy. If you find yourself going first you can allow your opponent to deploy and then move your Pathfinders 6" to put them in range/line of sight of a unit they wouldn't have otherwise been able to hit. Anyway, there are now very few Networked Markerlights meaning that Pathfinders are your best choice of boosting units.
Another key change is that Seeker Missiles must now be fired by the unit that owns them but can also be fired without markerlights. This is worth bearing in mind as, for example, broadsides could fire their railguns and seekers at a flyer at BS3 whereas pathfinders would need to hit with BS1 and then the broadsides would fire at BS5, the combination of which is worse than firing them normally. Most of the time then you're better to not bother using marker hits for seekers. The exception to this would be when using a Sky Ray or Sunshark marker (that has Skyfire) to fire a seeker from a vehicle that otherwise wouldn't have hit the flyer.
Anyway, assuming a squad of 10 pathfinders you're going to average 5 hits. Against ground targets seekers are an option but otherwise the best uses are Scour and Pinpoint. If you get lucky and score two hits against a flyer then Scour will help broadsides prevent flyers from Evasion. However, you're probably better increasing their BS. Scour is, of course, also good for fusion/plasma weaponry against marines. In its old incarnation a cover save was reduced by a point per markerlight but now 2 hits is enough to remove even a 2+ cover save (take that Shrouded plaguebearers). Pinpoint is no longer limited to BS5 by the way so feel free to make your Fire Warriors BS6+ depending on how lucky you were with your hits.
There's been much hype about the ability to improve Snap Shots, particularly for Overwatch. The thought of Fire Warriors Overwatching at BS3+ is pretty nice but realistically you'll only get a hit or two. It might therefore be worth using your Pathfinders' pulse carbines for overwatch/supporting fire instead, especially if there's an Ethereal around.
The key with markerlights is to plan out how you're going to use them. If you get 6 hits, for example, you have to weigh up whether its better to make two squads BS4 and Ignores Cover, give three squads Ignores Cover or make all three squads BS5. Situation will dictate this but planning is key.
Conclusion
Pathfinders are brilliant. They're not durable by any stretch of the imagination nor are they deadly on their own. They're going to be target number one though as the benefits they supply to the rest of the army are huge. I'll leave you with this image. Pictures 10 pathfinders getting 5 hits on guardsmen behind an Aegis line. A 12-man Fire Warrior squad could then wipe the entire squad out at 30" range with a little luck!
Sorry for the wall of text, I probably could've written a separate post on markerlights but thought it was a good place to discuss them in context. Once again, I'd love to hear your thoughts. Next up is the stars of the old codex the Broadsides, are they still up to the job with S8?