Sneak is an absolute must for dex rogues and (to a lesser extent) rangers. While skills that apply invisibility (WiS/Become Invisible/Make Invisible) are more reliable in that nothing can pull you out of invis except taking damage, they all have long cooldowns which means you can't really rely on them for anything other than either a one-off ambush primer or panic button when you're in trouble.
Sneak on the other hand has no cooldown and at rank 5 costs 1 AP. Best of all you can be rank 5 in sneak as early as level 7-8 with some luck. Get 2 levels in sneak (costs 3 ability points), and equip a helm and boots with +1 sneak (these are quite common from loot drops), and get the +1 sneak bonus from the Cautious trait. I've found sneak to be especially useful on Glass Cannon mages and rangers. You don't have to sweat the HP penalty from Glass Cannon when you can just have your guy enter sneak at the end of every turn and prevent enemies from targeting him.
As far as melee rogues being underpowered, it's not really true. Yes, they are harder to build and play than rangers because they don't have the luxury of sniping at enemies from 15 m away the entire fight. However, if you play smartly and support them with a mage they can easily match the DPS from rangers.
I will say, however, that comparing a pure dex ranger with a pure dex melee rogue, the ranger is much more versatile. A melee rogue is basically going to be a single-target DPS machine through and through, with little ability to fill other roles. Rangers can switch from single-target to AoE damage to CC to party support on the fly very easily due to access to special arrows and the greater variety found in Marksman skills.
