If the site folks are going to work with you to present a sure-fire application, then maybe they are worth the money. If all they are offering is advice, you can get that here free.
That said, I applied for the HSMP on my own. I was so certain I would get the visa, but the Home Office refused me and refused my requests for re-evaluation. Now I know that I should have included reams of back-up (organization chart, equivalent position postings, performance evaluations showing how my position evolved over the years, etc.) for my claim for points in work experience.
Knowing what I know now, I would have used an agency if they offered a no-visa, no-payment option. Good luck.