I don't personally ever intend on using Slaking for anything because I play games with momentum as my main purpose, but it still does make me very afraid when I'm facing it, simply because I don't build my teams with powerful Normal-type attacks in mind. Maybe if we still had Hippopotas, but we don't.
At least my main issue with it as a stall breaker is that it automatically loses to anything with Protect. Protect automatically generates a turn for whoever is using it against a Slaking, as the first time the opponent probably won't expect it and the second, you can get a useful double switch. Seeing as how Protect is one of the most common moves on stall teams, you're losing out on the best niche I can see Slaking having as soon as your opponent realizes what's going on.
However, with that said, I have had a few interesting run-ins with Slaking. I believe in one battle, my opponent had a wish passing Umbreon that would wish, hard switch to Slaking, Giga Imapct/Return/whatever, and that was a relatively effective strategy. The other was just a Scarfed revenge killer type thing. That battle was among the quickest I've ever had. I believe I killed two things to start, he brought in Slaking to revenge, I brought in a banded Azumarill (his fighting resistances being dead already), OHKO'd death fodder. Then my opponent brought Slaking back in, I used death fodder, OHKO. Azumarill, OHKO. Slaking, OHKO. Azumarill, OHKO. Slaking, OHKO. Azumarill OHKO for the win. Sure, the use of Slaking was cool, but didn't exactly work due to the fact that it lost the guy the battle. It was also a bit of luck on his part that nothing on my team could particularly take advantage of a free turn, and I'm sure most other teams can make use of it much better.
All in all, if you couldn't tell already, I don't think it really has a niche other than that gimmicky Mummy/Eject Button/Pursuit strategy, but I really don't think that something like that qualifies as a niche, especially when special attacks ruin it. If you want a speedy and powerful revenge killer, try Flygon/Raikou/Weavile/Crobat and enjoy NOT losing turns. If you want to use it as a stall breaker, Protect says hi. Stallbreakers by definition should be able to BREAK stall, not generate infinite free turns for the opponent.