Tesseract OCR is quite powerful, but does have the following limitations:
Unlike some OCR engines (like those used by the U.S. Postal Service to sort mail), Tesseract is unable to recognize handwriting and is limited to about 64 fonts in total.
Tesseract requires a bit of preprocessing to improve the OCR results; images need to be scaled appropriately, have as much image contrast as possible, and have horizontally-aligned text.
Finally, Tesseract OCR only works on Linux, Windows, and Mac OS X.