Fontella, I would say that the very existence of Plaid had a direct impact on the creation of the Welsh Assembly. If people hadn't voted for Plaid before the Assembly, then I think it's unlikely there would have been an assembly at all.
I'm not saying that Plaid "created" the Assembly. But the fact that enough people felt strongly enough to vote for Welsh nationalism, influenced things. If everyone in Wales had just voted Tory or Labour, I doubt anything around devolution would have happened. It's more about what the people who voted for them were expressing than what the party itself did.
And I think the Welsh Assembly has had some impact- lower MRSI rates due to hospital cleaning being taken back in-house, changed building regulations having a big impact on environmental sustainability. It's not headline grabbing stuff, but it will make a big difference over the long term. The building regulations stuff is actually a really good case study for how having control very some of the finer details can effectively create a much higher level policy, in a way that is actually enforceable.
Those things aren't directly down to Plaid, but the existence of Plaid means that the big parties have to pay a bit more attention to what people actually want in Wales, and so policies reflect that.
Both the Scottish parliament and the Welsh Assembly cannot overturn what happens in Westminster, but they can mitigate it's impact locally, just like the GLA did in London in the 1980s. And that can influence what happens in the rest of the country. It's also part of the reason the Tories are so keen on exporting the poor out of London this time round- they don't want a left wing London being a beacon of resistance/a place people can flee too when it gets too tough.