Your doubt is well founded. This is not a very good current source.
Draft48 shows the voltage across the two diodes (green), the voltage across the emitter resistor (red), the collector current (blue), and the power in the PNP (purple). Note that you must use a power transistor on a heatsink. These are plotted as a function of R2, so you can select a value that gives about 0.25A (6.8K).
Draft48a shows that it is a pretty crummy current source. R2 is fixed at 6.8K and V2 is varied from 0V to 15V to see that the current vries by 10%. Same colors as above. Forbid that we should vary the temperature....