achew: Pffft, then your friend is a n00b still, there is no M42 lens that does what the STF does.
I've had some interesting M42 lenses.
Industar 61 L/Z 50mm F2.8 (almost macro)
It's very small and cute!

Circular bokeh at F2.8, to a jagged star, to a star (F5.6-F8) and to a hexagon at F11 and finally a circle.

This is what the star-shaped bokeh looks like. Not easy to compose as you need small subjects. Focuses very near though.
Carl Zeiss Jena Flektogon 35mm F2.4 (almost macro)
Also small and cute, but heavier. It focuses to an amazing 19cm close!

Makes a great APS-C walkaround with good MFD. The Sony 30mm F2.8 DT Macro SAM however beats it in MFD (13.9cm) and can auto-focus, and
is cheaper (because the Flektogon is overrated.)
Peleng 8mm F3.5 circular fisheye
On APS-C, a diagonal fisheye with a bit extra (including the black corners.)

On FF, a circular fisheye with a minor trim off the top and bottom, but image quality on the edge isn't fantastic to bother about.

Now this is the Minolta 16mm F2.8 diagonal fisheye on FF. Note that there is no black corner. Diagonal fisheyes are easier to use than circular fisheyes.
So what fisheyes are there?
- Sigma 8mm F3.5 circular fisheye with autofocus and true full frame coverage
- Peleng 8mm F3.5 circular fisheye, manual focus, somewhat chopped top and bottom full frame coverage
- Sigma 10mm F2.8 diagonal fisheye for APS-C
- Sigma 4.5mm F2.8 circular fisheye for APS-C
- Sony/Minolta 16mm F2.8 diagonal fisheye for FF
- Vivitar 7mm/8mm F3.5 diagonal fisheye for APS-C
The Sigmas are a definitely better choice compared to the Peleng! They cost a bit more, though.
* diagonal fisheye - 180 degrees from top-left to bottom-right corner
* circular fisheye - 180 degrees from top to bottom
Other interesting M42 lenses- Takumar 17mm F4.0 diagonal fisheye pancake (small!)
- Jupiter-9 85mm F2.0 (tempting and very cheap)
- Carl Zeiss Jena 135mm F3.5 (popular lens, but well I like my auto-focusing Zeiss 135mm...)
The 28mm F2.8 lenses are generally optically weak so don't bother with those unless you can find the Zeiss Pentacon ones.
Manual focus is a lot easier with a split-prism viewfinder. However the only M42 lens I kept was the Peleng and it does not need precise focusing due to extremely deep DOF.
SHOfrE3zE: Yup it's the shop on the left, just as you enter - it's inside a phone shop. Not sure what the company name is there. They also stock most Tamrons for Sony mount including the Tamron 70-200mm F2.8!