i used some but they create a lot of errors and sometimes the mobile theme show instead of the website... plus it's create another structure of the posts links which give a lot of errors (according to google webmasters tools) so till then i never used one of them... it's my personal experience, maybe others had a success with...
Are the themes that you are using not dynamic/responsive already? I usually make sure that any themes I purchase are responsive. Maybe there is a difference between the mobile and responsive version?
That's what I was thinking. I don't think any theme authors would make their website without the responsive option any longer. With about 30-40% of traffic coming from mobile or tablets you have to have a responsive/mobile option for your website. This is a great way for your website to stand out over your competition too because I still see a lot of companies that do no offer mobile optimized sites, even some pretty large companies.