I've not used any of these but I can think of a few reason to treat them with some caution:
* differences in OO support. If your program implements OO features like inheritance or polymorphism then these may not be supported, or be only partially implemented, in the language you are porting to. If the converter just replaces language constructs, such as keywords, with their equivalents then you might find your app broken;
* lack of support for certain features e.g. PHP has excellent array handling structures while ASP has really good controls; and
* nothing is ever that simple
If you already have the logic for your app in place then porting it to another language might be a good excuse to do some refactoring anyway. Of course if the application is pretty small/straightforward then you might find these work as promised.