Without having direct experience with RestultFirst, I can't speak to their ability to obtain results, however a careful review of their site points out several serious concerns.
1. They carry a "Best in Search" "badge" from TOPSEOs. TOPSEOs sells badges for a price, they do not award these based on actual proven or independently verifiable success.
2. The site is chock full of grammatical flaws, the type of which generally come from outsourced solution providers where Americanized English is not the native language. It's actually quite poor when reading content in its entirety.
3. The claim of "white hat" SEO actually appears to be misleading at best. They describe in their services section, how they provide content - if the content they provide is written as poorly on a client's site as it is on their own, this itself would be a turn-off to many would-be clients or customers. It's highly unprofessional. So even if they could get a site ranked, there's just as much likelihood that many of those visits would be lost before conversion to clients or customers.
They then go on to state:
"the product & services of the client are made popular on social networking site and else, so as to spread the word of the brand. Numerous articles with targeted keywords are submitted on the submission directories which act as a link to the client’s site and this is another of the steps taken by the SEO Services Company to help attain a higher rank on the search engines.'
Ignoring the poor writing, they come right out and say that their primary link building method is what is actually considered a very low quality method of obtaining links, not high value nor SEO best practices.
In one of their "case studies", they state:
"In less than a month’s time, the website achieved rankings in the top 2 pages for keywords". Honestly, if the best they could do was get a site into the 2nd page of Google, that's not very good. And it's another validation to me, in my experience, that they don't really do high end work, but "just enough low quality methods" to get a site "sort of" ranked.
Now, they very well could do much more than I was able to ascertain, or than they claim on their site. They could even implement best practices, and not just "questionable" tactics. There's no way to know from their site however. And nothing on the site indicates they do truly high quality, long-lasting work that isn't in the "gray-hat" arena.
So - exactly what is their promise? How much does it cost? What does the guarantee specify, and what are the loop-holes in that regarding the possibility that some of the tactics they use might eventually fail due to search engines devaluing low quality methods?
Many questions need to be asked, and much written documentation would need to be presented for anyone to ever truly have the knowledge based confidence to want to hire them.
So please - be very cautious if you choose to proceed.