I'll try not to cover anything again that's been said because most of the good points about Eng have already been presented. I'll try to simply cover my own opinions on how it fairs relative to the other specs and also my concerns with it.
Eng rules low level sniper play. Its been shown that frag grenade is the definitive tool at low lvls. Which tree directly and indirectly buffs it the most? That's right, Eng. In fact, the other trees don't do jack for it. Meanwhile, Eng cranks out with Gearhead, Explosive Engineering, and (the awesome) Engineer's Tool Belt. Mid game MM and Leth pull abreast, if not ahead. Diversion/CG & Followthrough/Cull are gonna knock the **** outta IP (as much as I love it) and EMP Discharge; all of their skills start syncing up while we are just filling in some background effects. At 41 though, when we unlock PP, we flip the script. With its new improvements, its a complete game changer. All the little skills that were buffing this-and-that, suddenly are all working for PP. Increased dmg? Got that. Up'ed crit? In the bag. Reduced or free cost? Been there, done that. Combo with your oft-applied dot? Don't mind if I do. They keep running out of your Zone o' Death? Good thing you have a root & knockback+root to throw them right back in. Past this point, I truly believe that for the purposes of soloing (and I suppose leveling too) Eng blows MM/Leth outta the water, mainly due to them having issues with dealing with packs of mobs; Leth has too much set up and MM just doesn't bring enough area effects to the table. Meanwhile, their single target performance has never been shown to overshadow Eng.
The major issue I have with Eng in PvP is that, for lack of a better description, its burst is too burst-y. What I mean by this is that I'm perfectly capable of applying burst with minimal prep, but that I can't turn around and apply burst on another target. Instead, I have to wait 15 more seconds for my PP or EP to come off cooldown, and this problem was even worse before the IP refresh patch in 1.2. What this means is that I have to pull my punches a bit. I never go full in a target, I try to bring just enough hell down on them to force them to either retreat or make them a tempting target for a teammate who will finish/pursue them. The only time I go all-in is when I'm solo'ing a defensive point or far off to the side in a huttball match (or some other such). I do this because I never know when some Marauder will leap from the side and bring the full brunt of his abilities against me or some devious Merc has come from behind and above to flank me with his damnable Tracer spam. Its been well documented (anecdotally) that snipers lack any reliable ability to escape from a fight, so your only option becomes to delay until (hopefully) a compatriot can assist or fight off the attack yourself. So, it ends up feeling like I always holding something back, something in reserve and I am never living up to my full potential.
I hope that my rambling has made sense.