It's better than most give it credit for. Sandslash is generally less effective than Golem or Rhydon in its own right, but it beats them both one-on-one and is the only one of the three to outspeed Exeggutor. The trade-off is that Sandslash is the least offensive of the three, requiring a Swords Dance boost before it can really threaten your opponent, and the lack of Rock typing makes it a worse counter to Dodrio and Zapdos. Nonetheless, Sandslash is worthy of consideration for any OU team.
Sandslash is a Ground-type that can Swords Dance and beat a slightly injured Exeggutor. It comes in when you think a low Defense Pokémon is going to use Recover (Alakazam or Chansey) and it sweeps.
The problem with Sandslash, as opposed to other Ground / Rock types like Rhydon or Golem, is that it isn't a Rock-type, which decreases what it can achieve; it doesn't even have a great Attack stat. This makes Swords Dance a necessity, since Sandslash does not 2HKO Chansey or Starmie without it, but this, in turn, makes setting up tricky. Since it will also inevitably have problems against Starmie, Body Slam is there to paralyze it while it switches in, and also possibly wear down Exeggutor for Hyper Beam to be able to OHKO. Substitute is a possibility for when the opponent is fully paralyzed. Rock Slide is, of course, to hit the rare Flying- and Bug-types hard, which can be a huge pain if you are just trying to Body Slam them to death.
Mimic is a fun and viable option. You can use Slash if you are concerned about Sandslash's durability for setting up. Sand-Attack could work well together with Slash, Earthquake, and Substitute. If evasion moves are allowed, Swift is a usable option.
Checks and Counters
Starmie, Slowbro, Venusaur, Exeggutor, and Tauros all have sufficient Defense to switch in against Sandslash and threaten it with a 2HKO. Exeggutor is an especially good counter since it is resistant to Sandslash's main attack, Earthquake. Pokémon like Cloyster, Dragonite, and Lapras do well even with the weakness to Rock, since Sandslash doesn't get STAB on its Rock Slide.