<p>I know there are a lot of positive things mod-rewrite accomplishes. But are there any negative? Obviously if you have poorly written rules your going to have problems.  But what if you have a high volume site and your constantly using mod-rewrite, is it going to have a significant impact on performance? I did a quick search for some benchmarks on Google and didn't find much. </p>

I've used mod_rewrite on sites that get millions/hits/month without any significant performance issues. You do have to know which rewrites get applied first depending on your rules.
Using mod_rewrite is most likely faster than parsing the URL with your current language.
If you are really worried about performance, don't use
```
.htaccess
```
files, those are slow. Put all your rewrite rules in your Apache config, which is only read once on startup.
```
.htaccess
```
files get re-parsed on every request, along with
every
```
.htaccess
```
file in parent folders.