Rivendell Bow is effectively one of two things: 1) a less expensive Dunedain Cache that only attaches to certain characters or 2) for characters like Legolas or Haldir, a non-restricted attack boost. So while Rivendell Bow may not be the strongest weapon on the block, it is certainly a very good one.
Attach to a Noldor or Silvan character, or to Aragorn. Limit 1 per character.
Attached character gains ranged.
If attached character has the printed ranged keyword, it gets +1 during a ranged attack.
The Watcher in the Water #57. Tactics.
This is clearly a niche card since it can only attach to Noldor, Sylvans or Aragorn. My view is that it is useful for some of those niches and not very helpful for the other. For Silvans, most of whom are already ranged, Bow of the Galadhrim is almost always better because it gives more attack power.
For Aragorn and Noldor, this can be interesting. Aragorn isn't ranged and other than Trollshaw Scout neither are the Noldor. So if you're playing multiplayer and want ranged because your Aragorn or Noldor deck is providing the attacking power, you need this or Dúnedain Cache. And this card clearly wins that comparison when it is usable because it is cheaper and playable on allies. Furthermore, your second or third copies aren't dead draws if you have other ranged characters because these bows can give them an attack boost. And if you're using this, you probably should have other ranged characters in your deck anyway to support your ranged attack strategy.
I don't believe this card is very good unfortunately, or at the very least it is pretty much outclassed by Bow of the Galadhrim in nearly every way. I put multiple copies of this in my ranged Silvan combat deck but eventually left them out for Rivendell Blade.
The problem with this card, unless I'm reading the rules wrong, is the ranged condition. Just because the character gains ranged does not mean it is making a ranged attack. If an enemy engages you, this card is no good. If you have the ability to attack staging area enemies (with Haldir of Lórien), this card does not help you. Bow of the Galadhrim gives you +1 in that first case and +2 in the second.
Maybe there are some niches cases to this card, I'd say giving Aragorn or Elladan ranged if your party depends on it is probably the most likely. Perhaps you have an unranged Silvan like Mirlonde or something. You could give her this so that she can eventually get Bow of the Galadhrim.
I like to throw a copy of Rivendell Bow into fellowships that include Glorfindel as part of the more -focused deck. Since low-threat, high-willpower decks tend to want to avoid enemies, it is easy for Glorfindel's 3 to go to waste. Rivendell Bow is an easy way to remedy this. A similar concept holds true with Aragorn, who will often be questing and readying for the combat phase.
Overall, the card certainly has its place in the right deck/fellowship.