In short, I would say they’re worth trying out. As others have already said, they are pretty light reading, but I don’t think you should expect great literature from these books because that’s not what they were intended to be.
If you’re not looking simply to kill time, I’d recomend starting out with something by either Timothy Zahn or Michael Stackpole and seeing how you liked those before investing in any others. Really, you should probably start with the Heir to the Empire trilogy because those were the first three novels written, so there shouldn’t be anything in there that you won’t get because it was included in a different set of novels.
As far as individual books and series go, I’ll add my version of a Nightwatch Trailer-like List and use his scale… Ewoks=Stars? Very nice
Shadows of the Empire - Not the greatest SW novel I’ve read, I give it 2.5-3 Ewoks on the Nightwatch Trailer Ewok Scale.
Tales from Jabba’s Palace - it’s a collection of short stories, so it’s somewhat hit and miss, but I generally liked it and give it 3-3.5 Ewoks.
Tales from the Mos Eisley Cantina - another collection of short stores, but I didn’t like it as much as Jabba’s Palace so I’ll give it 1.5-2 Ewoks.
Tales from the Bounty Hunters - I liked the concept of bounty hunters in the SW universe and I particularly like Boba Fett, so I may like these stories a little more than the average fellow and I give it 3-3.5 Ewoks.
Tales from The Empire - third collection of stories (more on par with Jabba’s Palace) with a nice collaborative story written by Timothy Zahn and Stackpole earning it a 3-3.5 on the Ewok scale.
Tales from the New Republic - I don’t remember much of anything from this collection of stories, so I’ll give it a solid 2.75 Ewoks (2.5 if you’re a purist).
The Courtship of Princess Leia - Ehh… I don’t know if I’d go so far as to call it a horrible mess, but it certainly wasn’t that good - 1.5 Ewoks.
The Jedi Academy Series (of which I only read Jedi Search and Champions of the Force - I liked Jedi Search (novel 1) more than CotF (novel 3) and didn’t bother reading the second novel in the series so I’ll give it 2-2.5 Ewoks
Darksaber - I haven’t considered it good enough to re-read that much, so I give it 1.5-2 Ewoks.
Crystal Star - I have to say I liked this one, possibly because I read it while I was pretty young and wasn’t too discriminating. 2.5-3 Ewoks.
The New Rebellion - It’s a little off from the other novels and it helps if you’ve read some of the earlier ones because as I recall some of the characters featured in this one came from there, but certainly not bad. Another 2.5-3 Ewoks.
Children of the Jedi - I concur with Nightwatch Trailer’s assessment in that it didn’t have as much of the Star Wars feel as other novels, but it wasn’t too bad - 1.5-2 Ewoks.
Planet of Twilight - I believe this is the worst SW novel I’ve ever read, one of the few things I remember distinctly about it was that the whole thing felt odd and once again, not so much of that Star Wars feel. .5 Ewoks
Heir to the Empire Trilogy (Heir to the Empire, Dark Force Rising, and The Last Command (as Nightwatch Trailer pointed out, a Zahn series) - These are some of the best SW novels, imo, I give them 4.5-5 Ewoks.
The Corellian Trilogy (Ambush at Correllia, Assault at Selonia, and Showdown at Centerpoint) - Pretty good, I give the novels 3-3.5 Ewoks. Note: It’s helpful to have read these if you decide to read through the New Jedi Order series.
The Black Fleet Crisis Trilogy (Before the Storm, Shield of Lies, Tyrant’s Test) - Only second to the Heir to the Empire Trilogy trilogy, a very solid 4 Ewoks.
The Bounty Hunter Wars (The Mandalorian Armor, Slave Ship, and Hard Merchandise) - Another good trilogy, I put it in the 3-3.5 Ewok range.
The Hand of Thrawn Duology (Specter of the Past and Vision of the Future) - As with Zahn’s other works, these are two of the best SW novels I’ve read - 4.5-5 Ewoks.
Survivor’s Quest - Another Zahn novel, but I didn’t enjoy this one as much as I did his others but it was still good and it picks up towards the end. 3.5-4 Ewoks.
The Han Solo Trilogy (Paradise Snare, The Hutt Gambit, and Rebel Dawn) - I think these are geared toward people in their early to mid-teens, but as I was in that age-group when I read them and thought they were good, I give them 3-3.5 Ewoks.
There are also the Young Jedi Knights books, but these are definitely geared to young adults. That in mind, being a young adult when I first read them, they were pretty good, but probably not a good place to start reading SW novels.
As far as the New Jedi Order series goes - I have to say I didn’t like the idea of this as much as I did the earlier novels. Still, there were some good novels in there, but the drawback is that you do have to read most of them to follow the plot. Again, this is probably not a good place to start reading SW novels.
Of course, all of this is IMHO and YMMV and I also started reading these books when I was in second grade, so I did not have (nor do I now, I suppose) really discriminating tastes - I was just looking for something entertaining to read and I believe that overall, the books met that goal.
I realize that my ratings are fairly arbitrary and I apologize if that detracts from their usefulness, but I haven’t read most of these novels in some time (between several months and a few years) so I don’t precisely recall the parts I did and did not like. That having been said, the books really aren’t all that vastly different from one another - a crisis appears and the gang from the movies cleans it up, so my ratings are simply a reflection of how much I enjoyed the novels and that will be fairly subjective.