It's not a kiddie coaster, it is a true mini-coaster that is good for kids, teens, and adults if that makes sense. It's worth a ride, but as others have said you can do it without a FP and not much wait if you are willing to get up early or stay til close to park close. It is beautiful at night and a lot cooler back there as well. It is, IMO, perhaps closest to BTMR in MK, definitely speedier and more coaster-like than SDMT.
He does it compare to Goofy Barnstormer or SDMT as a "kids coaster"
On the "intensity" of Slinky:
For people who are scared of roller coasters, there is definitely a lose your stomach feeling at least twice that I definitely was not prepared for.
I wanted to make sure to write this because I did much reading on the ride before riding it, and definitely got a different impression about the ride then what it was actually like.
My impression from "research": Super easy, kid coaster. No drop feelings. Laughable speed.
I went into the ride without an ounce of fear. I was super excited to ride it. We were put in his tail section, and I felt scared for a second because the back of a coaster is the most intense, but then I got excited because I felt it would make the ride more fun as everyone said it was such an easy roller coaster.
On the first drop, immediate stomach dropping feeling. I was stunned! My boyfriend grabbed my arm b/c he realized I was scared, but I wanted it back b/c I thought maybe putting my arms up would make it more fun and less scary. It didn't help lol. So on the next drop I thought if I put my arms up AND screamed, it would be fun. So I did that and nope, still hated the stomach drop feeling. When we got to a slow down part I said "Absolutely no way am I getting on rockin rollercoaster!" LOL. (He and I hoped I would get on it for the first time that day).
So it DEFINITELY has the stomach drop feeling. For comparison this is how I feel about other Disney thrill rides as a wimperooni:
Space Mountain: LOVE IT. I only get a slight stomach drop feeling on the last "surprise drop". The drops are fun, surprising, and don't produce the stomach feeling for me. That being said, I haven't gotten back on it b/c I'm scared I'll magically get the stomach drop feeling...? Lol, I know, weird.
Test Track: Favorite "thrill" ride at Disney. I LOVE the speed. It's so exciting. No stomach drop feelings on this ride. As long as you are prepared for the speed, it's super fun. One of the most fun rides I've ever been on.
BTMRR: No stomach drops for me on this one. Fun but I don't ride it because it gives me a headache from all the jerkiness.
Frozen: Yes, I'm including this! lol. For years, I was scared of Maelstrom b/c I thought it was a real log flume ride. I finally road it as Frozen and learned it's not

. For anyone scared of drops, the first backwards can be a little surprising but once you ride it at least 2 times, it's just super fun and not scary. No stomach drop on this ride. The real "drop" is basically nothing, and the only part that produces a sensation is the backwards portion. But it's just like a "whee"!
Flight of Passage: Depending on how convinced your brain is of what's happening as you're on the ride, you may get the stomach drop feeling. If it starts to happen and you don't like it, just close your eyes and it immediately goes away.
Rides I have not ridden because I do not enjoy the stomach drop feeling (my poor boyfriend rides by himself):
Splash, Tower of Terror, Rockin Rollercoaster, Seven dwarves mine train (I'm scared of that one drop that you see when looking at the ride. I think it's the one as you come out from seeing the dwarves).
For reference, I also have ridden the following rides on a vacation to Six Flags to see if I could enjoy roller coasters: Nitro (yes seriously), Medusa, Viper, Rollin Thunder, Log Flume. Believe it or not, I used to love Medusa and the Log Flume, but as I got older, nope.
ALL of that being said, As much as I hated Slinky Dog, I also loved it. It's SUCH a good ride. Like really good, smooth, fun, well themed, just SO SO FUN! So even though I overall didn't personally like it, I think I'm going to get on every time I can until I learn to love the particular stomach drops on this ride. We'll see!
TDLR: For anyone scared of the roller coaster drop feeling, from one scaredy cat to another, yes, it does happen, and yes I did hate the feelings BUT I loved the rest of the ride. The speed actually was pretty fast and that was so fun. My boyfriend who rides thrill rides said he felt nothing so I think the majority of people who ride intense roller coasters will say the same. For those who don't, the feeling is definitely present. (EDIT: I possibly also had the feeling b/c I was in the very last car which is always the more intense part of a coaster).