Reflections -- NEX-5N & Tamron Adaptall 135mm f2.5. Click for larger.
The Tamron Adaptall 135mm has a few variations. The Adaptall 135mm f2.8, which is the most common, and there is also a white coloured 135mm f2.8. These two lenses have different coating on them, an indication that they were made in different periods. And of course this 135mm f2.5, which is a higher end of the 135mm lens. Tamron engraves "Close Focusing" on this lens to indicate close focusing capability, which has a 1.2m minimum focusing distance, where as most 135mm lenses are at 1.5m. This feature does come in handy for some quick close-ups.
Maple Seeds -- NEX-5N & Tamron Adaptall 135mm f2.5. Click for larger.
Like most earlier Adaptall lenses, which are very well made. This is no exception, with very good material used. Optically, I think the Pentax SMC 135mm f2.5 has a slight edge in sharpness, especially at wide open aperture. The Pentax is also a heavier lens and a little bigger too. Wide open, there isn't much to write home about; it's about average, but stopping down a little improves sharpness nicely. If you are not pixel peeping, you shouldn't have any complains.
The Pipe -- NEX-5N & Tamron Adaptall 135mm f2.5 @ f2.5. Click for larger.
The 1/3 of a stop of more light isn't really a big deal compared to the f2.8 version, especially with today's digital cameras with Über high ISO capabilities, but since manufacturers made this lens as a premium lens in the 135mm focal length, the build and material used is better than the cheaper, slower f2.8 counterpart. I think it's a good lens for the 135mm focal length since it's relatively cheap, and you can adapt it to pretty much all digital camera systems with the right mount.
Sample of close focusing -- NEX-5N & Tamron Adaptall 135mm f2.5.