My main concerns are: Is it fun solo? Is it good for playing with a party of friends? Is it good enough without expansions as I don't want to spend to much on it, and finally, does it fulfill a role that Descent (true dungeon crawl), talisman (fun crazy adventure), and Dungeonquest (a hybrid of talisman and descent) don't fulfill already?
Runebound is not my favourite hit on the table because:
1) it's quite long unless you don't play with 2-3
2) it has a longer turn dynamic with less interaction between heroes
3) I most often win this game, I think I've about a 16 games out of 20 winning score (same 3-4 players every time)
But it's a great game indeed, I like to play it here and there, not as often as the games you mention (which I also own).
It is a fun solo game, if you use the Doom Track or alternative means to give you a limited time to win the game. Without this, your Hero will eventually win the game since improvement in solo game can be endless. I played a lot of games solo to learn the balances and timings behind the game and this perhaps gives me an edge while playing with others; when you know Items & Allies, Basic Challenges, movement issues and how the difficulty progresses throughout the game, your gameplay becomes significantly better and you'll never complain very much about luck.
It is good to play with friends, but I recommend max 4 players. 5-6 player games tend to be overlong and boring, since you wait 4 turns before playing and a Turn usually takes 5 to 15 minutes, depending on player experience and action opportunities. Usually it requires more time than Talisman (even though an experienced player can claim victory when affordable and knows that he has only advantages in trying to win) and surely less time than Descent.
The base game is good enough for the first 7 maybe 10 games, but after that you need something fresh. Don't worry, since the Runebound expansions are usually card based expansions that change the setting and the adventure of the base game, or add additional stuff to the Market. Although not every expansion is to be found on sale now, there are still lots of them to choose and the Essentials Box looks like a cheap and valid choice. Big box expansions bring the adventure on a different map with lots of new rules depending on the setting/environment. I recommend those expansions only if you like the base game a lot and want a radical change.
The mechanics have nothing to do with above mentioned games. Talisman is an old style game and nothing of it is to be found in Runebound (no cheap PvP, no random movement, no random draws, no random object acquiring; there's still a degree of randomness but it's very limited in comparison). With Descent and Dungeonquest, the only thing in common is the Terrinoth setting, so be prepared to learn a completely different set of rules. I think Runebound perfectly fills the gap: it's quite a strategic game, despite being a lot more realistic than Descent (which is a 100% strategic dungeon combat game) and has a story/adventure flare that other FFG titles don't have. If you're an English-speaker you'll appreciate flavour text more than me and my fellows.
Hope this helps you in your choice!