My parents had/have a Credit Shelter / Bypass Trust.
They set it up in 1999, along with some other agreements that the attorneys said was necessary because Idaho is a community property state. I read the additional documents and never really was convinced of their necessity. I'm sure my parents paid several thousand dollars for these documents to be put in place.
My Mom died in 2016. After consulting with those same attorneys, it was recommended we do a TEDRA agreement. That cost $16K and took the better part of six months and killed quite a few electronic trees.
We now file federal and state income tax returns for the B trust annually. This required a CPA the first year for another $1500, but now I do because I can read and follow IRS instructions if you give me enough time. And it's mostly now just putting in this year's numbers on this year's forms. Although it does take me some time to collect, generate, print, collate, sign, mail, receive, deposit, and transfer the 26 different trust tax documents and spreadsheets each year. I also make an effort every year to figure out how much income to shift to my Dad's personal return in order to minimize taxes.
In terms of basis, the assets that went into the B trust got a step up in basis to their value on the date of my Mom's death. I don't believe they will get a further step up in basis when my Dad passes away.
In terms of estate taxes, OP and others might want to read up on the DSUEA. It requires a timely filing of an estate tax return (Form 706), and that's a long and complicated document (which probably requires a CPA and $500 to $1000) in order to basically check one box on like page 17 or something. But it may provide part of the functionality of a Credit Shelter Trust without having to go through the rigamarole of actually implementing a Credit Shelter Trust and doing everything we're doing now.
There has also been discussions of just automating the DSUEA election somehow where you can get it without necessarily filing a Form 706. But I'm not holding my breath for that law change.
...
I'd like to further add that I'm mildly annoyed at my Dad's CPA. When I suggested we file a Form 706 to preserve my Mom's DSUEA, his answer was that my Dad didn't anticipate his assets growing to exceed the estate tax exemption and it would be a bother to do something that probably wasn't necessary. Because he was the CPA and I was not, I let the matter drop. Now, it seems like it would have been a good bargain to pay that $500 or whatever and fill out the form in order to protect against the estate tax limit being dropped during my Dad's lifetime and costing his estate orders of magnitude more than $500 in estate taxes.
...
Bottom line, I used to think that "high quality" attorneys and CPAs were engaged in our situation as much as we were and would really do the absolute best, especially given what we paid them. Now I'm thinking that the moral hazard / agency stuff mentioned on this thread is a much more powerful force than I had reckoned, and that truly nobody cares about your finances as much as you do.
I suppose I may get burned with this attitude the other way, where me failing to write up a bunch of legalese and paying CPAs a kilobuck or two may cost me in the future. In fact I probably made that mistake in my divorce proceedings. But right now that's the way I'm leaning based on my experience so far.