Pros
They can capture objectives, Infiltrate, Stealth, Sniper Weapons.
Cons
Pricey.
That's Rangers in a nutshell. The question you will have to answer is "Are they worth the cost?" For me, the answer is yes. For you, it really depends on what other units you are taking. With the Eldar, your unit selection should compliment each other.
Let's go over their gear, stats and abilities...
Ranger Long Rifles are 36" range, SX, AP6 Heavy 1, Sniper, Pinning.
The SX means they always wound on a roll of 4+, regardless of the target's Toughness. "Sniper" also allows for Rending on a To Wound roll of 6. These boys are long range stationary shooters (BS4 is nice for this too).
Masters of Stealth
This gives Rangers the Infiltrate, Move Through Cover, and Stealth special rules. Infiltrate is always welcome in a Troops choice. This will allow you to position yourself as already holding Objectives that look to be out of the way of where the main battle will be (Please put them in cover, though). Move Through Cover is icing on the cake, allowing you to re-position your Rangers should you feel the need. Ideally though, your Rangers won't move. Stealth improves cover saves by 1. Given that 5th Edition has made most cover saves 4+, this means that Rangers enjoy a 3+ cover save! That is extremely impressive, and one of your main reasons as to why they're so expensive (just watch out for Flamers ;P).
Pathfinders
Pathfinders is an upgrade. This will improve your cover save by 2 instead of 1. That's a 2+ cover save, most of the time. 2+ cover save! These guys are amazingly hard to route when hunkered down in cover. Now, the downside of Pathfinders is that it costs 5pts/model. "So what," you say? Well, they're 19 pts/model to start with. Wait, what? You read that right, they're more expensive than Space Marines even without the Pathfinders upgrade. Do not make this decision lightly.
Ok, now that we know what Rangers are all about, we should compare them to other appropriate Eldar Units (i.e. other units that can capture objectives).
Dire Avengers
Given the Dire Avengers' 18" guns, our Rangers are still useful due to a difference in zones of operation. Dire Avengers should be capturing objectives midfield and in your opponent's Deployment Zone. Rangers are better left further away from the possibility of being assaulted in close combat. Put your Rangers on objectives in your DZ and objectives out of the way of battle.
Guardians
Guardians can hold any objective, depending on how much support you give them. You can equip them with close combat weapons and have them hold midfield-enemy DZ objectives, or give them a platform and have them hold your DZ, out of the way, midfield, whatever really. They may not all be able to shoot, but they're boots on the field and only cost 8 pts/model. That's less than half what a Ranger costs (and merely a third of what a Pathfinder costs). Still, at long range, a Guardian Squad doesn't have the potential for amount of infantry kills that a squad of Rangers does.
Jetbike Guardians
The tables have turned a bit here, in that Jetbike Guardians cost more than Rangers (22 pts/model compared to the Rangers' 19) Unlike with the Guardians example above, there isn't much of an overlap in usage here. Jetbike Guardians are taken for mobility. You will most-likely be using them mid-field, and having them Turbo-Boost to an Objective at turn 5. Rangers are a stationary unit by design.
Wraithguard
You heard me. If you take a full squad of 10 Wraithguard and attach a Warlock with the Spiritseer upgrade, you can take it as a Troops choice (another reason to stay away from Iyanden). Even given this is an EXTREMELY expensive option (381 points at bare minimum), there is still no crossover; this unit is for mid-field and enemy DZ objectives.
So really, it's between standard Guardians and Rangers for the long-range objective role. Guardians hold objectives well under fire because they're cheap and numerous. Rangers hold objectives well (if in cover) because they're VERY hard to kill without Flamers/assaulting when they're entrenched.
Painter's Note:
My Rangers above were painted with a layering technique (painted them before I got into my Washes craze). Armour is Royal Blue, worked up with a watered down Enchanted Blue (Lightning Bolt Blue will do too), then highlighted with Ice Blue. The yellow was basecoated in Bestial Brown, then worked up in multiple layers of diluted Sunburst Yellow (many, many layers. Yellow's a tough color to get nice and bright.) Leathers are Vermin Fur with a highlight of 2:1 Vermin Fur/Bleached Bone. The camo-cloaks are Bestial Brown, washed in Badab Black, highlighted with splotches of Snakebite Leather, then splotched again with Bubonic Brown.
