I got the Sigma TCs (both 1.4x and 2x) for the 150mm macro too. The Sigma ones are matched, so might be better than another brand. The Canon are way more expensive.
With the 1.4x, you still get AF but over a reduced range equivalent to I think 1x magnification overall. To get to 1.4x you need to MF. With the 2.0x, you lose AF totally. I did do a comparison with and without them elsewhere in the forums if you want to try searching for it.
In practice I regularly use the 1.4x as the keeping of AF makes it more usable for non macro too. The 2.0x I rarely use but if you go aperture one stop below max the quality is still good.
_________________Canon DSLRs: 7D, 5D2, 1D, 600D, 450D full spectrum, 300D IR mod
Lenses: EF 35/2, 50/1.8, 85/1.8, 135/2+SF, 28-80 V, 70-300L, 100-400L, TS-E 24/3.5L, MP-E 65, EF-S 15-85 IS
3rd party: Zeiss 2/50 makro, Samyang 8mm fisheye, Sigma 150 macro, 120-300 f/2.8 OS, Celestron 1325/13
Tinies: Sony HX9V.