Suggestion

Which extensions exactly?
Make sure that you turned on legacy mode because some of them work fine with legacy mode on.

It's always recommended to test with the free trial prior to purchase.

SOBI2 works with legacy plugin turned on but it does not give url's like the old version. The url's used to be like,

sobi2=>business_directory/category_name/name_of_item

the newest version is like

option,com_sobi2/sobi2Task,sobi2Details/catid,4/sobi2Id,3/Itemid,55/

What happens when J1.6 comes out?

"Joomla! 1.6 will have a Legacy Mode to support the changes made relative to version 1.5, but will not longer support any extension designed for Joomla! 1.0.  We feel this strikes the best balance between encouraging developers and site owners to move forward and reduce the strain on the maintenance of too many versions of code."

The the extension doesn't work for you by some reason.
Check which version of Sobi2 you have and compare with the version from readme.txt

Joomla 1.59 SEF Advance 1.9.6 and SOBI2 RC 2.9.1.0

And what does it say in readme.txt for sef_ext.php I created?

SOBI2 RC 2.9 ( www.sigsiu.net )
This extension is provided for free to all SEF Advance customers.
It is not supported and there are no guarantees it will work for future versions of SOBI2.

Please read the documentation and help files. I get tons of these "support" issues daily. It's not bad support but I just can't repeat things individually for everyone, that's why I wrote the documentation and FAQ.

Now, extensions will be updated when I get some time but I prefer to write it for the STABLE version and not RC as they are constantly changing.

Besides, SEF Advance supports router.php files for Joomla 1.5 native SEF. If the SOBI2 author would at least support the core Joomla SEF with this file, if he now doesn't want to write sef_ext.php for SEF Advance and other SEF components, it would work in SEF Advance too.

I wasn't asking for support.

What I made was a suggestion  to let people know what works with what, like in a table layout so we don't have to waste our time downloading unzip and read. It's not a criticism of your support.  Just a way for new customers to quickly see if it's for them. Joomla has it's own SEF functions that work rather good as well as the free component 404SEF. I thought there would be a bigger benefit in purchasing a commercial component.