I know most people won't be able to take full advantage, but man, if you can, getting money into a SIPP or other pension, especially via Salary Sacrifice, is just insane.
Say:
40k gross salary, and here is the kicker - you need none of it to live on because you have spare cash sitting in a savings account or whatever.
Normal salary will be 30.6k assuming no student loan. If you can put all that into a pension, the govt will top up by 25%, so you will get in your pension 38250. Not bad - you get top up on money you never paid tax on.
With salary sacrifice to the max? The minimum you can reduce your income to is minimum wage. On 40k assuming full time, that means 16k.
So, you salary sacrifice 24k, employer pays in the the NI they haven't had to pay the government of roughly 3k = 27k total.
Your take-home on 16k is just over 14k. You can only put a maximum of 40k in from all sources; you have 27k already, so if you put in 80% of the remaining 13k the government will do the rest - so you put in 10,400, the government puts in 2,600.
So you end up with 40k in your pension, plus just under 4k left over.
For a gross salary of 40k, you have received 43,850!
Yes I know pension withdrawals are taxable... except you get 25% tax free, and then there is a ten year gap between being eligible to take money out of a pension and the state pension starting up. If you are likely to have everything else in an ISA by that point, you have no taxable income -> you can take the full personal allowance from your pension tax free (assuming you've already taken 25% tax free). Ten years at 12.5k = 125k; even if you have 3-4k a year to come from there sustainably that's fine, the state pension is only at most 8.5k.
Phew. That's an increase of more than 40% on no pension contributions at all (and even more if you have student loans you're not intending on repaying...).