First of all and above all - STO: Legacy pack is not your standard expansion pack. It is NOT needed whatsoever to enjoy Star Trek Online to its fullest experience. This can't be stressed enough as you will probably see people everywhere complaining about the price which they actually don't have to pay.
STO is completely free to play and you can gain everything included in this pack playing the game itself (though it must be said it will take a lot of time).
What it does do, it adds a lot of stuff for Romulan characters in the game. A lot of stuff that you will be not be able to use on day 1 of your gameplay, but will became useful while leveling up your Romulan / Reman.
So is it good? Yes, every Romulan can find something interesting in there.
Is it necessary? Absolutely not, I personally bought it to support the game I have been playing for 5 years.
Would I buy it? Well I did, on sale for a fraction of the full price and that is also my recomendation - play the game to see if you like it and when you do like it, know what you want from the pack, then wait for it to go on sale :)