I think swim tools - pull buoys, paddles, fins, snorkels, etc - can be good, and I've definitely found some benefit using them. However, I think you need to be careful and limit how much you use them or they become a crutch, especially the pull buoy. I use the pull buoy some, but I try not to use it a lot. I think a lot of triathlete use the pull buoy too much, partly because they read somewhere that it mimics wetsuit swimming. That may or may not be true, I don't know, but I do know that using it too much makes me lazy. I find that I don't engage my core as much when using the pull buoy, so I try to focus on that when using it and since the buoy is taking the kick out of the equation I try to put a lot of focus on my catch and pull (it is a pull buoy, afterall) so I'm not just going through the motions and making swimming easier by using it.
One thing I do like the buoy for is days I'm completely smashed from other workouts. If I'm seriously struggling in the pool because I'm really tired, I might grab the buoy and throw on some paddles and work on my catch/pull and see if I can still get in some quality work. Fortunately, those days are pretty rare.
Here are 4 of my favorite swim tools:
Finis Tempo Trainer
Easily my favorite. This thing changed my swimming. I use it mainly for pacing long sets. Right now I'm swimming my 1000s at a 1:25 pace so I set this to beep every minute twenty-five and then I get feedback the whole time. Chasing the beeps keeps my mind occupied so the long sets aren't so boring, and I know instantly if I'm falling off pace. It was pretty eye-opening the first time I used it. I started out way too fast and completely fell off pace by the halfway point. I've gotten a lot better at pacing using this. A LOT better. Once I get comfortable at a pace, I bump it up one second and work on that pace for a while. It's a good way to slowly build your pace, and before you know it you're several seconds per 100 faster on your long sets. You can also use it to work on your stroke rate, which I'll start doing soon as I begin to build toward my first half ironman of the season.
Finis Freestyler Paddle
I have some hand entry issues. I turn my left hand out and then pull it under my body, and I angle my right hand a little sometimes so when I start my pull it's slicing through the water rather than catching a lot of water. These paddles are helping me work on those issues. They're a form building paddle rather than strength building. They're designed to come off if you don't swim with good form. I find they do help, and I've been using them in my warm up and cool down lately to help me focus on those issues and get in touch with good form prior to starting my main set. They're fairly small paddles so they don't increase the surface area of your hand much, so I don't see much risk of shoulder injury with these. I've never had any shoulder issues with any paddles, but I never swim hard with paddles.
Finis Agility Paddle
Can you tell I like Finis products? I just bought these, and although I haven't swam with them much yet I really like them. These might quickly become my go to paddles. They're kind of a mix between the Freestyle and a strength paddle. They're designed to force good form, but they have a little more surface area than the Freestyler. These don't have any straps so if you don't swim with a good hand entry or catch, they'll easily come off. All you do is slip your thumb through the hole, and you squeeze the paddle a bit with your thumb to help keep it in place. From there, it's water pressure that holds it against your hand.
When I first put them on, with my very first stroke I angled my right hand a bit and these immediately pulled away from my hand. Instant feedback. They take more concentration than regular paddles with straps, but that's a good thing. And without straps, you can take these on and off very quickly so you could work them into a set that doesn't have much rest and still hit your intervals. I'll probably use these for my paddle workouts rather than my Speedo Contour paddles (good paddles, but I love paddles that force good form) and I think I'm going to rotate between these and the Freestyler for use during warm ups.
Speedo Ankle Band
This is a cruel little swim tool, but a good one. Like the Tempo Trainer, this can really be eye opening. I haven't done tons of work with bands yet, so I can't really comment yet on how much they've helped me improve (if at all) but I think they're a good tool and now that masters is over and I'm swimming on my own I'm working them into my sets on Mondays and Fridays. Right now, I'm doing 4x50 on 1:15 with bands only (you might need to start with bands and a buoy) right after my warm up. I started out hitting the wall in about 44 seconds and have managed to bring that down to 40 so I think I'm going to tighten up the interval to a minute and see if I can start coming in under 40 seconds. Then I'm going to increase those to 75s and then 100s. A good stroke rate helps and so does engaging the core to keep your legs toward the surface (see goofy video).
So those are my 4 favorite swim tools right now.
Here's a great blog post from an outstanding coach, Joel Filliol: http://joelfilliol.blogspot.com/2012/01/most-popular-post-on-this-blog-is-is.html
They're all great tips, but I think #21 is my favorite and it's probably the number one thing I don't like about masters programs - they never repeat workouts. We repeat bike and run workouts all the time, and can easily see our progress when we do, but for some reason swimming always needs a new workout. I love repeating sets so I can watch my times and see if I'm improving. It keeps me motivated.