Some are improving rates for existing borrowers - not just chasing the new - and even for some savers, while one society has gone as far as to make "war" preparations in case of a hostile takeover bid from, for example, a bank.
Meanwhile societies this week called for government to tighten laws preventing short-term speculators from gaining from windfall payouts. "Our industry does not want to be hijacked by the bounty hunters, the speculators and the asset strippers," said Ken Culley, chief executive of the Portman society and chairman of the Building Societies Association.
Two big societies - Northern Rock and Yorkshire - this week improved rates for a total of 200,000 existing mortgage borrowers in moves aimed, they said, at emphasising the benefits of being with a building society, which is owned by its customers, rather than a bank. The idea is that a building society can return more of its profits to customers in the form of better rates.
In what it described as a loyalty bonus, Northern Rock knocked 0.29 per cent off its mortgage rate for borrowers of seven years standing or more. For most of those affected, that takes them down to a mortgage rate of 7.7 per cent from the society's (and the general norm's) standard variable rate of 7.99 per cent. The average saving on a typical pounds 50,000 home loan is pounds 12 a month.
The Yorkshire reduced its standard variable rate to 7.85 per cent, in what is claimed to be the first such move by any society to benefit all of its existing variable rate borrowers.
A number of smaller societies, including the Cumberland (which two months ago cut rates by 0.75 per cent to 7.24 per cent for borrowers of three years standing or more), have made similar moves.
In addition, there is anecdotal evidence that societies in general are increasingly prepared to offer special discounts to those who threaten to remortgage with another lender.
Some savers are also benefiting. Along with their rate improvements for borrowers, both the Yorkshire and Northern Rock also improved rates for a number of savers last week. The Yorkshire set a minimum interest rate of 2.75 per cent on its savings accounts, while the Northern Rock scrapped a number of old accounts whose rates were less than those on similar accounts currently on offer.
Overall, customers are gaining from a happy coincidence of the desperate price war for new mortgage borrowers, fears of losing existing customers, and concerns to head off criticism that savers and borrowers are not deriving as much benefit as much as they might from societies' mutual status.
This follows the windfall gains of pounds 500 upwards for customers of the Halifax, Leeds, National & Provincial, and Cheltenham & Gloucester - as the former two merge and convert into a bank, and the latter two are taken over - and continuing speculation about moves by, in particular, the Alliance & Leicester and the Woolwich.
For customers, this cascade of cash and apparent consumerism may seem too good to be true. "Breathtaking - quite extraordinary," says Frank Kraus a specialist building societies' lawyer who has worked with a number of societies that have introduced loyalty bonus schemes.
Our table shows what each of the top 10 remaining societies - those that have not announced any takeover or switch to bank status - is doing to emphasise the benefits of being with a mutual and, by comparison, what each society might be worth in terms of windfalls in any takeover or switch to bank status. These biggest remaining 10 are the best bets for opening savings accounts in the hope of landing a windfall. As well as the Northern Rock and Yorkshire, the Bradford & Bingley, the Britannia and Birmingham Midshires are all looking at loyalty bonus schemes for customers. These might take the form of rate improvements or cash-based "dividends" or bonuses.
Interestingly, many of the societies in the table that are not planning such special bonus schemes are also considered the most likely to be taken over or to convert into banks.
Speculation that the A&L and Woolwich in particular are good bets for savers looking for windfall payouts of pounds 500 or so is strengthened by the comments of the head of a smaller society that has already improved rates for existing borrowers: "Their agenda is an open secret. They wish to keep their profitability high to get the best possible price [in a takeover or conversion]".
Also notable is that two of the most ardent proponents of mutuality - the Nationwide and the Portman - are not at the forefront of these special moves. Mr Culley at the Portman said his society aimed to give value through consistently better rates over time rather than via a specific bonus. He contrasted his society with many of the other big ones, which earlier this year raised minimum opening balances to pounds 500 to deter speculating savers. While saying it did not wish to attract speculators, the Portman had kept its minimum opening balance at pounds 100 and raised the interest rate on that balance to a relatively healthy 5 per cent.
Our table also shows societies' positions on uncompetitive old accounts - sometimes called obsolete or closed. Most societies have cut back on this rip-off, whether by shifting savers out of the accounts, improving rates or advising savers to move. But many savings-rate improvements of this kind have been offset by general rate cuts elsewhere.
The same jaundiced view could also be applied to some of the special mortgage-rate cuts. In this latest move, Northern Rock favoured borrowers of seven years standing or more. But earlier this year, the same society imposed redemption penalties on borrowers of less than seven years standing taking their mortgage elsewhere. It should also be noted that a handful of lenders - Direct Line, for example - still manage to undercut the new standard rates on offer from societies favouring their existing borrowers.
Building society Potential Out-of-date accounts Special bonuses
Nationwide pounds 530 Yes, but made competitive None planned
Woolwich pounds 690 Yes, but trying to convert savers None planned
Alliance & Leicester pounds 590 Yes, but point out better rates available None planned
Bradford & Bingley pounds 600 Yes Next year
Britannia pounds 630 Scrapped Announcement this year
Northern Rock pounds 500 Scrapped 0.29% off mortgage rate for borrowers
of seven years' standing
Bristol & West pounds 390 Yes, but made competitive None planned
Yorkshire pounds 740 Yes, but made competitive Min savings rate 2.75%. Mortgage rate
reduced to 7.85%
Birmingham Midshires pounds 400 Scrapped Planning
Portman pounds 520 Yes, but point out better rates. Plan to close None planned
Source of windfall figures: UBSReuse content