Similar to other Treefolk cards like Dauntless Dourbark and Dungrove Elder, the Kalonian Twingrove has its power and toughness decided by the number of Forests you control. The difference is that every time it enters the battlefield, it makes a token of itself. Two creatures for 5G that can potentially both be extremely big is a strong card. The trouble is that it simply makes big tokens.
Unlike Dauntless Dourbark from Lorwyn, which costs 3G to play, it doesn't gain trample as long as you control another Treefolk. Armada Wurm is a mythic rare from Return to Ravnica that's a 5/5 Wurm that makes a copy of itself, but the Wurm has trample, and as does any tokens that it makes. Because Kalonian Twingrove doesn't have trample and doesn't have any sort of evasion, it and its token friend can be blocked by 1/1's all day.
Also, unlike Dungrove Elder from Magic 2012, the Twingrove doesn't have Hexproof. That inability to be targeted by opponent's spells and abilities made Dungrove Elder Standard playable in mono-green decks. Still,simply having two potentially really big creatures coming from one card is fairly solid.
Kalonian Twingrove may have not become a big name in the Standard scene at any point, but it did actually see play in some top decks. There was a 3rd place Mono-Green Standard Deck at PPTQ Milano which used 2 copies of Kalonian Twingrove. It also featured Chord of Calling and Nissa, Worldwaker. Another Top 8 Mono-Green Standard deck with Setessan Tactics and Hunter's Prowess also had 2 copies of Twingrove.
Where Kalonian Twingrove still shines best is in Commander, where enter the battlefield effects are abused early and often. There are plenty of other ways to reuse enter the battlefield effects in Commander, such as Conjurer's Closet, Skybind, and Panharmonicon. There's also token duplicating effects with the Populate mechanic and cards like Parallel Lives, Primal Vigor and Doubling Season.
Green is already a color with some ridiculous card in Commander. Paired with its other Treefolk friends in Doran, the Siege Tower and Sapling of Colfenor decks, the Kalonian Twingrove is a nice complement. Kalonian Twingrove also sees a good amount of play in Omnath, Locus of Mana and Nissa, Vastwood Seer Commander decks.
So how much is Kalonian Twingrove worth? Twingrove's price has never been higher than $1 and usually has stayed between $0.25 and $0.50 for most of its life. It's not one of those cards that should ever be worth a bunch of money, but it's definitely one worth having around. It's a bulk rare with some actual value.